Instead of NASA approach, which without a change in funding isn't going to happen within any foreseeable perspective, let me look at the SpaceX approach.
Combining ISRU (In-situ resource utilization, essentially off-Earth production of fuel) technologies and airbraking one can make huge savings in fuel.
Let me describe the missions in a similar way as one goes with Kerbal Space Program, starting from the end and adding more stages, more boosters, heavier rockets as one gets towards the launch.
Apollo was all-in-one:
- Earth reentry and landing: airbraking; a robust capsule with a heavy heatshield, no fuel.
- Earth return from lunar orbit: ~1km/s with the above, plus an engine and a tank for that. (also, propelling the ascent vehicle, point 3., even though it was unnecessary)
- Lunar ascent - Lunar Ascent Module. A small lightweight, paper-thin-walled 2-person craft with ~1km/s of delta-V, enough RCS fuel and capability to meet the capsule in lunar orbit.
- Lunar descent and landing - a Lunar descent stage of ~1km/s carrying the above with all its fuel to a soft surface touchdown. Also landing legs and a bunch of equipment not needed on ascent.
- Lunar insertion - Propelling everything so far by about 1km/s into low Moon orbit, using the tank and engine from (2.)
- Lunar transfer - propelling same as above from LEO, again same engine. About 4km/s. This was done by last stage of the launcher, with the above payload in a massive fairing ("stage IV/B").
- and 9. Finishing orbital ascent with same as 6 + acceleration to near orbital speed with stage II (alongside with a launch escape system) and lifting it all up and giving initial kick with stage 1. For nearly 10km/s total.
Every time you reach next point multiply the mass so far, including fuel so far, by delta-V needed and you're getting the scale, size and mass of the next (actually previous) stage. As you can see this grows into the enormous Saturn V with a tiny capsule in the end. Nothing reused. All the fuel including last return burn carried and propelled all the way. Lots and lots of fully disposable stages.
Now let's try the same for BFR mission to Mars and back.
- Earth landing. Powered, using small pressure tanks with cryofuels carried in BFS inside its large tanks. Only several hundred m/s. although the vehicle is large, and includes heavy heat shielding.
- Earth reentry. Using airbraking, so free (using heat shielding on one side of the rocket)
- Earth capture. Airbraking, free, same as above.
- Mars orbit departure and Mars-Earth transfer. About 2.5km/s out of the BFS main tanks.
- Mars orbital refueling. A launch of 1-2 BFSs from the surface to bring fuel to the return vehicle, followed by their return to Mars surface. While free for the return vehicle, double or triple the cost of everything from now on; you need to get them to Mars!
- Mars ascent. Main tanks, using fuel made from Mars in-situ resources. About 4km/s but essentially free other than initial cost of the equipment. Before that step, the tanks in all the vehicles can be empty, the only mass so far brought to the surface is 3 or so BFSs, not a drop of Earth-made fuel.
- Mars landing - powered, small pressurized tanks, several hundred m/s.
- Mars reentry - free, aerobraking.
- Mars capture - free, aerobraking.
- Earth-Mars transfer - about 4.5km/s out of BFS main tanks. (x3 rockets)
- Earth orbit refueling. Now the numbers really grow, because we need about 6 BFR launches to bring all the fuel for point 10. And they can't be too far apart in time, due to boil-off, so you need a fleet of at least 9 of these (3 to Mars, 6 for fueling them up). Maybe slightly less (you may relaunch the first ones by the time the 4th or so finishes fueling up the ones in orbit.) So, this step is not adding anything, just multiplying the subsequent steps by 7.
- and 13 About 9-10km/s - Earth ascent and orbital insertion, either with payload for travel (food, life support, science, crew) or just payload of fuel, split between the BFS and the BFR booster (reusable).
And that's it. One Mars landing means about 9 BFRs, each of which is about the size of Saturn V, and likely costs similarly. The difference being that everything either is recovered, or remains at the destination as permanent infrastructure for further reuse. After building these 9 BFRs, if nothing crashes, you can send as many missions to Mars as you like and they will cost about 18 tanks of methane and oxygen worth each, in exhaustible resources. Meanwhile, all that's left from Apollo is some museum pieces and you're not going to reuse them.