If BTC is gonna be used in future, it not gonna cost anything, because the BTC is gonna be the value. It not gonna be like Bitcoin cost 50,000dollars... Bitcoin not gonna cost anything because Bitcoin IS the value. But that's probably not gonna happen. We not gonna pay with BTC but with stable coins like USDT...
If it's the main currency then it's going to have a cost and the same as its value. Although in the past, I've always believed that bitcoin can kick and replace the traditional fiat system and it's going to be the main currency of all, I think we're still far from seeing that into reality.
But I disagree at the last part, we're paying with bitcoin, we're using bitcoin just as the fiat, dollar, stable coins and any other medium of exchange because it's the main purpose of bitcoin.