Costs around 30 bucks. You get a new gasket, small tube of aquaseal, sandpaper, and instructions. Not much, but cheaper than sending it off and easy to do at home. Something many will eventually need.
Suggestions: I found the cardboard ring method the kit recommends to be clumsy. Due to the excess fabric of the suit, I could not get even pressure when “setting” the gasket to the suit and had to spot patch several spots the morning after. A better method would be a multitude of wood blocks and small clamps to hold the two sides firmly together, you’d need quite a few but I can’t think of a more secure way.