This one could be tricky but interesting. You guys should consider markets modeled after your poll approval markets but regarding candidate fundraising. For example, Will Ted Cruz have raised greater than $5 million dollars for his presidential campaign committee in the March quarterly filing? The question will always be resolved by whatever is posted by the FEC.
While it'd be interesting to include Super Pacs in this, we can reasonably expect it'd be too hard to keep track of which PAC is backing who as many will be popping up.