@Russell has the right idea. Pricing is about economics, what the market will bear. What it costs to produce and launch is only one factor in that equation. Let's look at it economically: dollars per kilogram to orbit. Here are the numbers for payload to GTO. First in 2014.
- Falcon 9: 4,850 kg to GTO for \$61.2M or \$12500/kg
- Falcon Heavy: "Up to 6,400 kg" to GTO fro \$85M or \$13281/kg
And here they are in 2018.
- Falcon 9: "Up to" 5,500 kg to GTO for \$62M or \$11270/kg
- Falcon Heavy: "Up to" 8,000 kg to GTO for \$90M or \$11250/kg
Note the cost per kilogram is about the same and have converged.
Also note the 2014 pricing on the Falcon Heavy is $85M for "up to 6.4mT" but it is capable of putting 21mT into GTO. How much would that cost? They don't say.
The key comes when you look into the potential reusability of the Falcon which SpaceX is gambling will drive the cost of launching stuff to LEO down to the magic \$2,200/kg (\$1000/lb).
SpaceX has indicated that the Falcon Heavy payload performance to
Geosynchronous transfer orbit (GTO) will be reduced by addition of the
reusable technology, but would fly at much lower launch price. With
full reusability on all three booster cores, GTO payload will be 7,000
kg (15,000 lb).
7,000 kg is awful close to the marketed "up to 6,400kg" price. They don't have that reusable part working yet, but they're using every Falcon 9 launch to test it.
The other part is volume. SpaceX plans to build a lot of these rockets and build them cheaper each time. By using the same basic technology for all their rockets, they further increase their economies of scale. And since launches have a fixed cost, more payload per launch might mean cheaper launches... though fuel fraction remains a tyrant.
SpaceX has claimed the cost of reaching low Earth orbit can be as low
as US$1,000/lb if an annual rate of four launches can be sustained,
and as of 2011 planned to eventually launch 10 Falcon Heavy and 10
Falcon 9 annually.
So there you have it. The Falcon Heavy's posted price is for significantly less than its maximum capability and banking on yet to be proven technology and a steady stream of contracts to drive down the cost of launching stuff into orbit. It's the vehicle SpaceX is banking on to drive launch costs down to encourage the sustained space economy they need to get to Mars. Wait, Mars?
Running a successful business is nice and all, but SpaceX is one very, very rich man's dream to colonize Mars. The Falcon Heavy is not the vehicle to do that, that will require a whole new engine, but it is the vehicle to get to Mars cheaper than anyone else.