Might be worth doing a self build so you can use up any useful parts from your existing machine. CD drives and the like could be carried over easily, modems, graphics card , sound card, hard drive even. Depends on the spec of what you have and what you would like.
A local PC shop may also offer you a quote that includes your reused parts.
I'm building a base unit for the father of a mate and I'm reusing his CDRW and floppy. Saves him about £30 just but better than nothing.
You should be able to buy the parts for a new base unit for well under £300 including the OS.