- Is my cost analysis fairly complete and realistic?
Remember to budget for the PSU, and the per-node cost of getting the power to the PSU, a UPS to keep it going, a means of extracting the power again after it has been recycled as heat, the physical space, the rack, the Ethernet switch + cabling, something to talk to the console (whether serial or KVM) and a way to press the reset button remotely.
All of which costs money, and some of which you may already have or be able to scrounge. But it has to go into your budget: if you spend everything on the nodes, you'll end up having to buy the cable out of your own pocket...