You can do a search on eBay for current Pentax 50mm lenses available for bidding:
http://photography.shop.ebay.com/i.html?_nkw=Pentax+50mm&_sacat=3323&To%2520Fit%2520Camera%2520Brand=Pentax&_dmpt=Camera_Lenses&_odkw=&_osacat=3323&bkBtn=&_trksid=p4978.m270.l1313
You can also do a price check of recent actual transactions by refining your search to completed listings. You will need to login first to view this listing.
http://photography.shop.ebay.com/i.html?_nkw=Pentax+50mm&_sacat=3323&To%2520Fit%2520Camera%2520Brand=Pentax&_dmpt=Camera_Lenses&_odkw=&_osacat=3323&bkBtn=&_trksid=p4978.m270.l1313
You can also do a price check of recent actual transactions by refining your search to completed listings. You will need to login first to view this listing.