Unless it's been stored somewhere quite damp it's probably entirely functional; blow it up(compressed air or a tyre pump if you have it to keep the inside dry but I'm not sure it matters much) and leave it overnight, if it's still up in the morning it's fine. Check over the gas mechanism and bottle for rust, if no rust then you're good to go, deflate, fold it up and go hovering. The only bit which expires is the soluble capsule that fires the automatic inflation, unless it's going crusty from damp it is probably serviceable. If you really want to keep it orthodox buy a service kit and fit the new soluble capsule unit.
You are right though, for the cheaper lifejackets service kits seem to be 2/3rds of the price.