Musk could technically pay for it, his wealth is right around $6 to $7 billion (33m shares of Tesla worth $4.8b; $1b in SolarCity, and SpaceX is worth at least a billion to him, plausibly more), but it'd wipe him out.

There are reasonably a few individuals or groups of people in the world that can very safely spare the resources to build it (such that paying for it it wouldn't materially damage their business or ability to run their business, or otherwise tank their stock, put them in the poor house, and so on).

Bill Gates (Cascade can raise $10 billion quickly), Carlos Slim, Warren Buffett (the street already knows he's going to liquidate), the Koch brothers together could easily do it, Ellison could do it with leverage (he'd never sell that much Oracle stock in one go, and he earns enough annually in dividends to pay off the loan), and the Walton clan (but they'd be very unlikely to coordinate on such a thing).

For almost anybody else, the liquidity or leverage it would take would be overwhelming or very risky at the least.