I started with $300, and at the time it bought me 1 ETH. But since then, I have introduced people to the markets who have bought in when Ethereum was at a price of between $700-1100. I think a minimum of $100 is best, means that you've got money to lose and you can also make some. It's all relative, but yeah, $100 would be my shout to make some decent money. Obviously the more you put in, the more you can make, but consequently the more you stand to lose too.