It depends

For ERC-20 USDT, the fee is much more higher. There is another which is the one built on Tron blockchain, but I do not know if it is swappable, but there should be a decentralized exchange which is TRC-20. I do not know much about altcoins.

USDT built on Binance Chain (BC or  BEP2), if existing, the fee will be much lower than that of ethereum and you can compare the fee as the Binance coin (BNB) on BC.

On Binance Smart Chain (BEP20), the fee is much more lower, BNB and USDT are both on BSC, the fee also is both low.