Ya, that is a good cost, I recently paid about $250 for a aftermarket cat and muffler designed to increase gas mileage and put it on myself. I noticed that I get much better gas milage driving in the mountains (2-3 MPG) and just a little better gas milage driving in the city (.5-1 MPG). Since my car had 200k miles I figured getting rid of the originals was a good bet.
The downsides? The muffler is about as loud as before (although its a deeper, controlled rumble now, before it was just loud and rattily) and the jeep sometimes vibrates when in neutral at a stop light. I don't like the vibrations, I might just need to tighten up some screws on the dash to resolve, we'll see.
Good luck with the testing.