The actual $ cost of the coins is not relevant. Cardano is about 1$ but it's a top 5 currency and is not as likely to double in the future as say waltoncoin which costs 10$. What you really need to look at is the Marketcap because that takes into account the supply of the coins as well as the cost. Look for coins under 10 million marketcap. That will essentially get you what you are looking for.