I want to say that I read somewhere the 21" rims w/tires are close to $100 lbs each. Yikes. Makes sense with all the extra heavy material you need when the rime is several inches further away than a standard tire. They are also sort of wide. C=d*pi. A 21" diameter tire has 20% more material than a comparable 17" - at least, 'spoke' members may get thicker as they get longer.
