The battery manufacturers put a 5 year expiration date on them. They would probably last 10 years, but would you want to take a chance?
The raft repacking cost is a mystery to me too, but when I look at the paperwork that explains what they do, there is certainly some labor involved and some equipment that is replaced. I would guess a valise style is somewhat cheaper than a canister as they don't have to open the case and then seal it up when they are done.
But like the battery, do you want to take a chance? If a seam has opened up or the tubes have somehow been punctured, the raft isn't going to save your life. Can you be sure that the CO2 cannister is still full and that it's valve isn't corroded? I remember seeing a report about a sportfisher that went down a couple of years ago, and his 13 year old never serviced raft worked fine and saved his crew. He was very lucky, but I'll be that if he is still boating, he gets his raft serviced at the factory recommended interval.