Retail prices aren't necessarily a direct reflection of cost. Many factors will determine the retail cost, most importantly what the market will bear. Tube amps seem to do well on their voodoo/mojo appeal, and prices reflect that, just as they do with Gibson guitars, for example.

Popularity of tube guitar amps, and competition seem to be affecting retail prices in favor of the customer (at least a little), with more models being offered, and more units being made and sold by major brand manufacturers.