Cardano, as opposed to Ethereum, has a fixed transaction fee (currently 0.17ADA). There has been no problem with this fixed fee and no-one tried to cheat. Probably it is because there is not much to get from cheating, because it seems to me that stake pool could cheat the fixed cost if they wanted to:
For example, Binance holds a big part of the stake. So, they could offer you a refund for say 50% of your fees if you do your transaction with them (instead of sending your transaction to the network, you send it to them). So you would have to wait a little bit (but not so much) but you would get lower transaction fees. Binance would receive more transactions to handle, but if they have at least roughly twice more transactions it is profitable for them.
If the network becomes saturated, stake pools will be able to ask people to pay a subscription fee to see their transactions processed.
I think there is not much of a mechanism to prevent that to happen. Is there?