Hard to say without looking at it.
A single cat conv. was about $200 last time I had one done (Paid $250 for cat and muffler.) Just checked, first hit on cat was $68.
The bolts can be replaced. It is most often studs not bolts. They can be drilled out and replaced if needed.
If the pipe from the manifold to the cat is ok; the old cat can be cut off and a new one sleeved or welded on. No big deal for a decent muffler shop.
Heck, the idiot that did my Cherokee welded the cat on rather than get one with the correct flange.
Rusted nuts can be cut off then old studs cleaned up with a die. No big deal if it is accessible.--
Embedded Systems Consultant,
SHDesigns home - DIY Welder