If a friend asked me how much he should invest in bitcoins for his first investment I would tell him to use as much money as he can afford to lose. 30 USD would be better than 10 USD, and 100 USD would be better than 30. It all comes down to his financial situation and what free money he has available. The best situation to buy bitcoins would be to make constant purchases every month of 50 or 100 USD at least. Another important thing is to remain realistic about the future growth of Bitcoins. When you buy only 30 USD it's not like you expect to make a profit of 50 USD in a few weeks. The times of making high returns overnight are over.