Adds up to approximately $1000, for what I used.  I intend to build another prototype using only 2 GPU's ($60), a mini ITX board($105), i5($160), 8-16 GB RAM($90-170), 1tb HDD($40), and an expandable mini ITX case ($95) that would not look out of place in your living room. With incidentals this should be less than ± $700, which is very much inline with offerings from initial "SteamBox" vendors that do not incorporate virtualization. 

Testing can be done with much less than what I have used and I would assume someone attempting to follow along would have some baseline of computer parts to draw from. A combination of VT-d capable board & CPU can generally be had for $200-250, and maybe $40 per minimal PCIe GPU.  Combine this with another system you cannibalize and you can build a Hydra for $2-300.