I believe filter prices margin are high. I guess manufacturers price them not from the cost point of view but from the value i.e. buyers willing to pay $$$ to protect their expensive lens. Filters, unlike lenses, can be used on any lenses with the same diameter so the market is larger.
A 'behind the envelop' calculation:
Take the Canon EF 50mm F/1.4 lens - Approx $600. Elements : 7.
Average cost of each element (lens) $85.
which include other costs such as packing, alignment, lens tube, auto focus motors, aperture blades etc
Approx prices of a 58mm UV filter (from CS forum price guide) : Tokina ($10) , Hoya HMC ($25), Nikon 52mm ($35), B+W MRC ($70).
which include other costs such asl plastic case, metal screw mount.
Well, it is up to the buyers to decide what is the balance between the cost and benefit.