Your question is not clear? maybe you are not aware of some information. by the way, USDT has been using different networks/Blockchain it was used in Bitcoin Network, Ethereum, Tron, BSC, and also in main Binance Network, But I will answer base on your question it depends on the network, but I guess USDT on BSC is the cheapest transaction fees, second is the USDT on TRON network.
I guess the opposite is the right answer; USDT is much more cheaper in Tron network then for BSC smart chain. USDT for polygon is also so cheap but it's not widely used out of the Matic network itself which is not that famous as Tron or BSC.
It's always up to you how you aim to proceed the swap as it's so important to right choose the network. USDT is a stable asset in almost all the networks but the difference is always in the transaction fees. Please avoid using ethereum main network, you will lost a great portion of your savings just for paying the network charges.