How much does it cost to take a converter off a car?

Essentially nothing, if you do it yourself. However, this is almost certainly a violation of anti-pollution laws.

Unless you want the exhaust to just pour out underneath the car, there would be a small cost for a section of pipe to reconnect the exhaust manifold with the muffler and tailpipe, but since you're already breaking the law that might not be a big concern to you.