TLDR: there isn't and it doesn't matter. What people wish and what they do is more important.
For some reason I was believing it was migrated from worldbuilding and was about to write how so so sooo sooo much is wrong with the question, but it seems brain damage on my side, so I will say - quite wrong perspective, so as certain degree of op's laziness as there plenty of mars holly warriors out there and discussions on that topic.
Zubrin brings 3 major incentives, I do support only one of them, finding or making a step in the finding of the origins of life. This question, or more results of it have multifacet fundamental implications related to long therm survivial of humankind. I do not buy second basket, I mean a different thing but it sound too outlandish if described in few sentences, like the q - do we build death star or not.
It more to illystrate that subjective incentives behind people's interest about mars and activities on mars can be different and wast.
This just one example, but there are many more, and each human interested in mars can have it's own, as mars is a planet, it is big big big, so it not a surprise that there are many angles to look at that elephant.
Sorry for such an introduction, but it how I do things and it can't be helped. Here a litte bit more basics.
Mars is a planet, a planet, Karl. Yes, compared to earth it a bit smaller, but compared to any country it is big, and it even bigger if yout think about that there is no wildlife to preserve, I mean 100% surface can be used for whatever. It can host full cycle economy, full cycle of technologies, all supply chains whithin itself.
It does not get old, Mars is a plnet. It is not a banana republic. It means it can host as much industry as Earth. Selfsufficiency is 100% possible, there can be questions in how to get from initial nothing to that result, but there are no doubts about possibility of such result.
And here we get to few fundamentals
- at this point, writing the answer, I realised that you probably worded your question wrong, and it should have been - is there a self propeling, self greasing way to build colony on mars and develop it to a city size at least. An step by step road, where our investments balance to net zero losses with profits at each step, at least. Meaning good old capitalist way, when you do not take credits but invest your profits in yourself. The answer is yes, but how is a bit different question, I will touch some parts of it here but it more complex.
Whatever cash making thing you are looking on mars, there will be perod of investments, when one spend spend spend money and get nothing in return. It does not matter what is that cash making thing you imagine or wish. It also means whatever is a short therm or long therm goal, or idea behind the activiy, they all start the same by pouring cash in the thing, a lot of cash. And how long it will take before you start gain some returns, depends in ideas, and the ways it is done, but by no means it will be a short period time but more like decades, it takes 2 years for launch window to appear, how can it be fast. (There are unconventional approaches, but out of scope, not a constat g, just a different starting position for space activity)
A wrong thing number one is looking at mars as cash cow from the start, it not a cash cow, it money black hole.
There is nothing bad about that or unusual, same as the home/house one lives and have mancave there it is a place to live, and we work, many of us, elswhere to sustain it. To feed ourselfs and maintain the home.
In that sense one of the ways is to have a profitable business on the earth and invest dividends in your house, mars colony whatever. And in that sense StarLink is a good example.
Cosidering initial anoncement was like 500 grand per ticket, and goal like a million people, eventually, it is a potencial for 500billion investment, 500 billion business.
When you think about Amazon it not neessarly looks that big but it 2 General Motors or 3 Boeing size companies. And for mars the companies which deal with technologies are important as it needs to export technologies(knowhows how to make thigs, not equipment) from earth to mars to be able to produce technological goods for local consumption, in that regard SpaceX and Tesla are good examples, especialy those incentives of in house production, meaning having a whole chain of expertiese of converting source materials to end goods, at least taste of it.
Looking for sole/single incentive to relay upon to drive the whole train it won't do, it won't work for moon and it won't work for mars, space habs a bit different story, belters are on the border betwee yes and no. It won't work because the task is highly diverse and big, and income from a single thing won't satisfy you demands in most cases, cases regular/casual people talk about.
You invest what you have on earth and profits you have on earth, to open new perspectives for yourself, new possibilities. You buy yoursef an opportunity, and mars is goods/commodity and its business value is secondary. I'm not a big fan of mars second home, I prefer space habs, but problems with those major 3 or 4 directions(hi belters - moon, mars, space habs, asteroids) it pretty much the same. Where get the money and how to establish technological and industrial capacities in places of your choice. If one thinks to maintain supply chains between mars and earth, this one is clueless aboutn what it takes and what technoogies are as of today. Dital goods and services those are viable.
And it needs to understand that establishing industrial capacities in space is a fundametal problem so as one of the universal solutions for make things happen.
I had other aspects to talk about as well, but I think it good enough conslusion, a good starting point for anyone who think about those problems, so then let's leave it at that, as it is inconvinient to type on a tablet, so as it gets too big.
Plz some one run a spellchecker on the answer.
It seems I didn't gave any food at all, so I have to give at least some, here it is
cash cow - development of nuclear and fusion reactors, selling technologies or alike.
Energy on mars is one of the fundamental problems to solve, so nuclear and fusion rectors are one of the things which can solve the problem, and due absene of water and life and it radiation anyway - there is more freedm in therms of what safety measures are required and which are not and how you conduct testings and stuff.
Due the necessities and freedoms it may lead to more intensive and rapid development in those technologies, more installations more hours of proven work, more robust constructions - so mars can take leading edge in those technologies and keep it that way and license them for profit. License fees or other ways to gain profit on that knowledge, like mars inc which is certified to use those technologies on earth, and exchanges profits for what mars may need.
There are many aspects of life on mars which require special attencion and requirements, and some of the technologies can be exported.
In that sense mars may become a science hub, technology hub concentrating productive workforce which is located as on mars so as on earth, and channel those technologis in to profits on earth - improoving life on mars and getting money for that.
So that probably is your golden bullet and ladder in the sky - deveopment of technologies.
It may work because quite often for technologies to start and survive before maturing it needs sufficient incentive to develop it, mars has unique set of incentives, a different one from earths established economy and way of life, and portion of those important for mars technologie can be used on earth, after they are mature enough.
As an example, here on earth we can choose recycle trash or not, on mars it a necessity, and clearly they will be better at doing so at some point.
Or food production, robotic farm equipment, soil preparation, indoors growing, gmo's, fish growing .... just insanly big topic important for both places, but more so for mars, and sure they will be better at that at some point.
Medicine, another big big topic with higher importance for mars.
And those are things mars can't slak on, meaning strong incentive for real efficient and good managment, and any mistake will be on display very fast and it will cost lifes. Boeng, nasa can slak and spend resources god knows where, they won't die if there is no result, they can alow themselfs to be not focused, and if there is a result their life won't improve that much, if any change at all. Totally different environment and perspective on mars - you get result you have improvement in your daily life as the communiy as a whole - sense of purpose will skyrocket, testings is it good or bad idea will be fast and efficient.
Etc. Technologies is new gold.