Joe Biden's comeback is not only upending the presidential primary, but revealing new insights into the limits of money in politics and Biden's potential strength. This exclusive MSNBC special report documents each campaign's spending per vote. It exposes some surprising results, ranging from Bloomberg's pricey failure to revealing the campaign with most "return" on investment, as Biden spent just $10 per vote, less than his chief remaining rival, Bernie Sanders. By contrast, billionaire Michael Bloomberg spent over $200 per vote, raising questions about a potential ceiling on the efficacy of self-funded candidates.