I've been told that the cost depends on the age of the raft. They have a finite shelf life as do the contents inside. If the raft's reasonably new, then $300-400 is a good budget
number. If you need to replace any contents (especially if it's SOLAS rated) the costs go up. If the raft fails the basic inflation test, then it's time for another raft. I've been told that rafts older than 10 years are probably going to fail bad enough that buying
a new one is the only solution.
There have been news reports concerning criminal charges being brought against life raft service stations for faulty testing, theft, or other illegal acts. I've always tried to be there when the raft was unpacked and inflated. That way I get to see the condition and inventory the contents as well as ask questions. I try to be there when they pack it so that everything goes back in and they leave the sand and lead out.
Once you see all the stuff in the case, you realize that it takes experience and training to get it all back in, in correctly, and able to inflate when needed. Just as I'd never pack my parachute, I'd never pack my life raft. I don't have the training for either.