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I have seen several examples sell over the last 5 years on eBay, at auction, or through dealers, with prices apparently ranging from £80 to £350, depending upon size, condition, and whether they are signed with an 'R' cane. I agree with Nicholas: there is always an uncertainty between original asking price, and the final sale price!
I also agree that prices seem to be moving up for the rarer / better Riedel pieces - which is true as well for many antique paperweight genre, or course.
Alan