I guess how expensivee they are is a personal thing. They are sought after as 964 and 968 owners look for them to 'upgrade' their cars. A pair can be had for around 120 quid, if you are in no hurry. Douglass valley breakers in the uk had some last time I checked for someone.
Other breakers too in the uk have them from time to time (a place near Burton, but I forget their name PH Sportscars I think). There is a porsche specific breakers in Cali that had them on the shelf for those interested in the U.S.
www.type-911.co.uk sell second hand sets for 230 a pair.
I suppose if you fancy a set the best answer is to buy a copy of 911 & Porsche world and ring all the breakers.
I might try to find out how much a new set would cost, just out of interest...