What was the lift sent to the shop for originally? I can't think of anything the service company could have done to clog the cat-cats clog from impurities in the fuel (not using HD-5 grade fuel), overheating the cat (running the engine with a faulty cylinder) or physical deterioration (banging around from rough terrain, etc).

The quote doesn't seem excessive as cats are quite expensive- unlike a car where you can get generic cats- the cat for your lift is specific to only a few models- so when sales volume goes down, price goes up.

As far as buying a new lift- just understand that newer lifts (2004 & newer) are full of electronics which are expensive to maintain.