Around here you can get a Magnaflow cat welded in for $250 including parts and labor. I had this done on my wife's Toyota that had a persistent PO420 code with good results.

Aftermarket exhaust parts don't last-- about 3 years or so. If the rest of your exhaust system is in decent shape, you can just have the cat welded in at a good independent exhaust shop.

What condition is the rest of the exhaust in? Another alternative is to replace everything behind the cat with a genuine Volvo cat-back system, assuming you are planning on keeping the car for more than a year or two. This option would be way less than $1500 even including an aftermarket cat-- you can price the exhaust at online dealers like Tasca.