I think to earn at least a little, you need to invest at least $ 100. Smaller amounts are simply ineffective for investment.
If someone can't invest $100, even $50 will be enough, but the rates of return/total profit might not be that high. Actually it depends upon the individual, how much he wants as profit. If he doesn't minds small profits, $50 would be fine.
To start an investment, there is no need to wait for any particular amount to accumulate because starting investment is very important. If you start once, then you know how to increase that investment over the time if you're making a profit from the investment. If you start with the smaller amount then you risk also will be less along with the profit in that way you can understand investment better and can make a decision whether you want to increase your investment on any particular product or not.