Assuming it is not a relative, (which would just open a whole can of fucked up worms!) offer the groom a free session!!!
LOL
But in all seriousness, if you're still a student/young, I think 50-100 is plenty for a gift card. One from Linens and Things or one of those stores which sells frilly towels and crap for a new house/homestead for newly-weds would be best! Have fun at the wedding.