For the few exchanges that I use, trading fees are calculated using the base currency (where in a currency pair C1/C2, C2 is the base currency). For example, for pairs BTC/USD, the fees both for buying and selling are denominated in USD. I read that Mt. Gox used to have fees in BTC. Are there other exchanges out there who are calculating fees not from the base currency?
I use only two exchanges(kraken and binance), on kraken the fee charged is in base currency. but on binance it's different, you can buy BNB and use it as your base/fee deductible currency also you pay lower fees[1] when you use BNB.
[1]
https://support.binance.com/hc/en-us/articles/115000583311