average is hard to say....20-30lbs would probably be it though.
As you approach 1lb/inch you approach upper weight 'light' wheels, if you can get near 1/2lb per inch you are in the ultralight category.
Every 1lb on a wheel is bad, like 5-10lbs (20-50lbs when you consider x 4) on the car itself. Plus braking increases and all that (ever see someone with 24's on a SUV and stock brakes try to panic stop (a lot of panicking and not much stopping

).