Catbacks are very simple to do by yourself, especially if you know someone who has a car lift you can use. Anyway id guess 75-100, most shops seem to be 40-70 an hour. It took us (2 people) about an hour with no rush whatsoever and tracking down lost tools.
If you have basic mechanic know how i say go for it
Headers are another story, looking at 250-350 for installation and its alot more in depth. Still doable but a much longer/pita job
Only time my exhaust required welding is when i cut the cats out, axelbacks should be bolt on.