Only what I could have afforded to lose. Around $5k. Not a lot of money (maybe even too small an amount to overcome the friction of traditional early-stage fundraising), but I would have gotten a much better RoI than my indexed stock-market fund got.

>and how well would you have done had the start-ups went belly up?

Well clearly I would have done poorly in that case. But that is why I don't invest more than I can afford to lose in risky investments.