The minimum payout seems to be a cent.
You could make more money gambling with btc...and with less effort. Not that I would recommend doing this, either...
They aren't bad if you do them passively. Each site takes 5-10s so you spend about a minute every hour or two. The good thing is that as BTC goes up in value, so will your earnings. So what is worth 13 cents today could very well be worth dollars later on.