I don't have a Catalytic Converter on my car, and I bought 1. I bought a magnaflow one and it was like 90 at the exhaust shop, but the problem is that it is universal.

I looked on the net and I seen that a lot of the high-flow cats can start really really cheap to like 400$.

My question is what is the difference between cheap, expensive , and OEM?

And what is the point of buying an expensive one?

Are you sure you bought the catalytic converter and not a muffler? I can't imagine that a catalytic converter would cost you $90, more like a couple hundred.

Thats what I thought to, but thats what they said, and the exhaust shop said it was a high-flow cat...

I should have added that I have not changed out my exhaust system, so I have no idea as to the answer to your question.