I agree it shouldn't. There are lots of simple designs out there that are free to rip and put in a box for little money, and there should be some stuff on the market like that. The darkstar is a good example of someone overcharging for nice case work for an already existing design (for those who don't know, see: http://en.wikipedia.org/wiki/Gainclone).
As hifimanrookie pointed out not all designs are equal and you get into R&D costs, boutique parts, original design, etc. A lot of amplifier designers are on a quest to get the absolute best sound and are constantly asking how they can make something sound even better than their best work. This of course varies company to company.