if you trade intraday, interest is not charged. At least I think this is how it works in trading. If you hold your positions overnight - then only the leverage interest can be charged. But you are right, if there is a significant leverage, like it happens with AAFX, where the ration reaches 1:2000, you won't be able to profit much from your open positions as interest might be too big.
That will be true in case they charge some interest though.