OK so it looks like the Plug-In wheels weigh... 15" Prius Plug-In Wheel Weight : Prius Forum .net 18.25 lbs (bare wheel, no TPMS, no center cap, no valve stem, no wheel weights)
http://www.moviephotoforums.com/photopost/data/882/medium/IMG_1568.JPG 37.5 lbs with tire, thanks to Rockerdan
well i guess that explains why the regular prius wheels don't look as "nice": they are designed that way to be lighter.