I think, you might be needing a lot of funds when investing on gambling site because they have minimum funds to be accepted and its kind a risky move because its all or nothing and win or lose situation. However , gambling is not an investment but a game where money counts and not just a number.
The minimum amount required to invest in gambling site's bankroll is 0.01 BTC(some require even less) and it's not that risky, over long-run you will make profit. the more you'll invest the bigger returns you're going to get.