If they are truly getting tons of gifts from constituents, the $20 is low. Even if they don't get a lot $20 is too low. I would say $100 is a better limit, and that would be at the bottom end.
Way I see it, the real corruption in DC lies in the earmarks. I can't see the beneift in tracking the nickle and dime stuff. No senator was ever paid off in gift baskets ;^)