First of all, are you aware that USDT itself can belong to different chains like BEP20, BEP2, TRC20, ERC20, OMNI, SOLANA, MATIC, etc?. As far as I know, fees are charged for the type of token you have. If you have a BEP20 USDT, you will be charged in BEP20 BNB and so on. Based on that, to answer your question, BNB or USDT based on BEP20/BEP2 is cheap for swapping. I hope someone will correct me if I'm wrong, but this answer is based on my knowledge. I hope it could help