Service interval might be one year; might be three. Three years for newer rafts which are vacuum packed.
How much it costs depends a lot on what needs to be done, and what is inside your raft. The service itself is around 100 pounds in the UK at most places. I spent around 500 pounds having my Avon Ocean raft serviced. But that included hydrotesting the gas cylinder and replacing the trigger
head, something you don't have to do every service.
My service would have been even more expensive had I replaced the horrendously expensive
battery which powers the canopy light. Since I have strobes and torches in my grab bag, and the
battery was still good (after 10 years!), I decided to skip that expense. Likewise with the very expensive mini rocket flares which were in the raft's RORC kit. I have rocket flares in my grab bag.
Besides the Avon 4-man raft, I also have a Seago 4-man raft. The service station where I had the Avon serviced advised me not to bother with the Seago. Although highly rated, the Seagos have weak seams, according to the service guy, and are not worth servicing. They are so
cheap (about 500 pounds I think) that it's best to just bin them (or Fleabay them) after three years and buy a new one.
The Avon ones, on the contrary, according to the service guy, are good for a few decades -- built like tanks. Probably why Pete's was bought so eagerly by someone.