I think investing in gambling site for small investors are not going to be profitable at all as the profit you may get with it is really small. So the lower amount you are going to invest with it then the chance of getting high profit by investing with gambling sites are going to be low too. But if you are going to invest higher then higher chance of getting high returns.
It honestly depends, if you invest a small amount in a site with a small bankroll, you can still make profit. I invested a small amount (0.01 I believe) in moneypot when it first opened, and since it was new and didn't have many backers yet, I made a decent profit. Of course, you are taking the risk of investing in a lesser known site, but if you want to profit from a low investment, this might be something you might want to consider doing.