I've built three different cabinets myself, and I'd guess the cost was around $1200 per cabinet. It takes quite a bit of stuff: lumber, oak panels, drill, saw, hole saw kit, wood glue, clamps, nails, screws, good centrifugal fan, carbon filter, air-cooled reflector, 600w digital ballast and good bulb, darkroom louvers, ducting and duct clamps, white paint, brushes, paint thinner, casters, handles, 6" wall flange, screen, staple gun, sandpaper, wood stain, staples, caulk, caulk gun, and so on. Much of it is little stuff, but five bucks here, ten bucks there, and it definitely does add up. To be fair, I bought the highest quality equipment available on the market -- because after all the *other* costs, and after all the work, why cheap out? Not to mention, the blood and sweat and time was worth something.
So, I don't think their prices are *that* unreasonable. I think my cabinets are waaaay better, but they did cost more, even though I built them myself. I also bought a cabinet from Lowe's and modified it to create a 400w flowering cabinet out of it. That was cheaper, but it's also nowhere *near* as light-tight as the ones I built myself with that being one of the specific goals of my design.