If it has to be x86, I'd suggest finding something used on ebay or something.
If not, and their needs are low as you say, why not an EOMA68 computer card + micro-desktop? :) It only adds up to a total of $120, and it's upgradeable. It even consumes a whole lot less power, so they'll save money on electricity in the long-run (in addition to the long-term savings on upgrades).
