I blew my neck gasket in my Stohlquist about 4 years ago (back when the Stohlquists were the most bomber top you could get, IMO). I sent it in and they replaced it for about $45, including shipping. I'm pretty sure they did not leave extra material like John mentions. They did an awesome job considering I didn't trust myself to do that kind of repair on that expensive of an item, at the time. It did take about 3 weeks, though, a real bummer if you don't have a spare (or if it doesn't happen at the end of the season, as it fortuitously did for me).
"The world would be a better place if everyone kayaked."-Brad Ludden (Valhalla)
"You only get one chance to run a drop blind."-DD