She's already in debt, her current donor base isn't going to foot the bill, she isn't a billionaire (as other long shot "independents" who run campaigns in earnest have been) and while she might recoup some of the money after the fact through public financing it probably wouldn't cover the whole thing (my understanding is that "new party" candidates receive after-the-fact reimbursement based on the proportion of votes they win), the idea that she and Bill would risk both their personal fortune and their political legacy just to play spoiler (If she can't win her own party, she isn't going to win the general against the guy who beat her and republicans and she knows it). She'd also have to come crawling back to the Democrats in two years (I think) when she's up for reelection in the Senate, and something tells me they'd be less than welcoming, even if Barak won.