It depends on the exchange you use, every exchange will have a different price. For example,
https://poloniex.com/fees/ charges 0.15% to make an order, and .25% to take an order. In theory the transaction to buy BTC and change it to an altcoin, you are looking at .4% . Let say you want to sell your altcoin for BTC, you would make an order and have another .15% charged. Finally, you would have a fee to exchange BTC to USD with another exchange since poloniex does not have usd bank transfer. Now you pay a transaction fee to send BTC to another exchange and the fee to exchange to usd. That being said, fees do add up, but an altcoin <$1 has a much greater chance to jump up 50% or 100% then some of the more establish coins, and the risk/fees can be worth it.
