I believe the whole point of the phrase "sunday best" is to tell us that we need to take His religion seriously, that when we walk into His church we're doing so with as much reverence and respect as we can.
My daughter asked me this question too. I asked her if she would consider wearing an expensive pants suit when she got married. Her answer would probably match yours.
My wife gets all her sunday clothes at Goodwill and other thrift shops and she is regularly complimented on how nice she looks. It's a lot more work finding nice clothes at thrift shops but the price is worth it. Maybe give that a try?