What about the votes of the people you influenced in his direction? If just 2 other people turn to him as a result, your vote against him has been effectively negated. Perhaps the $2500 means more to you than the vote against Bloomberg? If so, that's fair. A lot of people could use that kind of money, and I don't blame them for taking it. Ironically, that speaks directly to the imbalance of political power caused by the existence of massively wealthy individuals like him.
But assuming you are OK with promoting a candidate in exchange for $, why not take money from a candidate you believe in, rather than one you plan to vote against? Is it because (just guessing) he's paying more? In which case, it seems a lot like he's just buying the primary.