The mountings for the bump stops are a welded part of the rear subframe. A new subframe (from Fiat) is around £650 I believe. There was someone on the forum a while ago who said they could supply them for a lot less than that. A new pair of rear dampers will only cost about £50 absolute max.
That's the parts prices (inc. VAT). A garage will take 1/2 to 3/4 day to get everything fitted. So £1,000 sounds a bit steep, but if it's a main dealer probably not far off the mark.
The alternatives:
Get someone to fabricate new bump stop mounts and weld them to the existing subframe. That will be a cheaper option, but you'll need to hunt around to find someone with the skills to take the job on. Also, if you've completely lost the old bump stop rubbers, you'll have problems getting new ones as Fiat don't sell them on their own.
or
Get another subframe from a scrappy and have that fitted.