Depends wither you need a catalytic converter. A new muffler, installed, will run $100-150. Exhaust pipes probably $100-300. New cat, $100-300. Chain muffler shops will often try to sell you more than you need, watch out there.
Sometimes you can find used cat-back exhaust (that's from the catalytic converter on back), it's usually a bolt-on job with minimal skill required. Junkyard prices will be about $100-150.