£200K divided by 350 machines = £571.43 per machine. Now let's say you want a 20% ROI, £40K divided by 350 machines = £114.29 per machine. So you are already at £685.72 without building a single machine. The case costs ~£100, so you are now up to close to 800 quid. The other parts, memory, hard disk, video card probably cost another £200. Somebody has to put it all together, give them maybe £50 per machine, plus another £50 at least for storage and other costs, so you are up to £1100 or so. The overhead may be far more than that, so you are likely talking about a £300 to 400 motherboard. Pricey, but within reason for such a dinky quantity...