How many miles on your car? Cats last about 150,000 miles.
You'll want to make sure your cat is actually the issue. Taking it to an autoparts store for a free OBDII scan can get you the codes to ensure nobody lied to you.
At rockauto, a bolt in cat's going for $188+shipping, and can be changed in the driveway with ramps and a couple hand tools everyone should own anyway.
You could shop around for cat itself and have an exhaust shop do the work. $35-100, plus labor... $100? So, $135-200 total.
$700 is far too much.
If you have to, bribe a buddy with some beer to come help you.
