After selling Paypal to eBay in 2002, Musk and his partners netted $1.5 billion. An unknown portion of that went to Musk, who promptly turned around and invested several hundred million into not one, but three enterprises: Tesla, SpaceX and SolarCity. Neither Tesla nor SpaceX are generating enough revenue yet to be self-sustaining. The last we heard from someone inside Tesla, the company was selling about 20-30 Roadsters a month for likely revenue of under $4 million, plus whatever it's getting from Daimler for Smart ED battery packs.
Apparently Musk isn't personally self-sustaining either, claiming that he has $200,000 per month in expenses and $8,255 in income, presumably from his CEO salaries at Tesla and SpaceX. Musk's filings also list only $650,000 in liquid assets. Given that disparity, it seems like there might be at least a bit of leeway to cut back a bit. At any rate, Musk claims that he has had to start taking personal loans from friends to make ends meet.
Needless to say, Musk's estranged wife Justine sees the family finances a bit differently, and the reality is likely somewhere in between. If Musk's financial position is, in fact, as strained as he says, he'll want to complete the Tesla IPO as soon as possible just so that he can cash out some of his position.
[Source: CNN Money, Forbes]