things may have changed but the last set of nos blue ones I bought cost me £100, (2 years ago)  which I thought was too much,  but I needed them 

,  and single ones usual sell on here for about £20 (its always tricky to then pair them up), its the large flange ones that command £100 plus sealed or unsealed..   

red ones in mint condition I would say about £80.....
sam