Spay/neuter costs vary widely based on location, vet's experience and size and age of the rabbit. For example, one of the vets I use, will spay a rabbit under 1 year for $95 plus the office visit (neuter is $75). That same vet charges over $200 if the doe is over 1 year. My other vet, charges close to $300 for a spay and simply will not spay a doe over then 1 year. (I have found that many vets won't do a spay on a doe that is over 1 year due to the amount of fat that builds up and the added risk due to extra time under anesthetic.) The best thing you can do is call around to vets with rabbit, not just exotic, experience. Be prepared with the rabbit's age and approximate weight.
As far as the benefits, there are many. Basically, unless you want to breed, there isn't a good reason not to spay or neuter your rabbit. Either way, you get rid of those nasty hormones. Some does are just grouchy after they reach sexual maturity. I once had a doe who was only happy when she was bred. Bucks, well, are boys and boys will be boys. Neutering will sometimes help if you have a buck (or doe) who sprays. If you want to bond bunnies, the easiest pair to bond are a spayed doe and a neutered buck. You lose the risk of uterine and testicular cancers...the list really goes on and on. Most of the benefits of spaying or neutering any animal apply to rabbits as well.