You are right, there are many cheaper alternatives out there, including DIY. But:
First, you will be buying a product from a serious company with an outstanding customer service. The amps sound good, the workmanship is excellent. There is no delays or BS with this company. This is not a situation were you buy a product and 6 months down the road they will be obsolete because there is a new iteration of the item. Headroom has a established product line. Ultimately, the choice is yours.
The DIY community is a very valuable asset that keeps providing great amplifiers, (and maybe Tyll could shime in here,) and also keeps manufactures under a pseudo "evolutionary pressure" to evolve along with new designs as well. This is a very good thing to everyone. But again in terms of production costs and profit margins, it is very difficult to reconcile the two spheres. more on this below...
Second, and more financially oriented note, running a company that has full time employees, with benefits, etc, involves having a profit margin that can accommodate for such a set up. Then there may be some advertisements costs involved as well.