You can get a decent power supply for around £40.
This one from Antec, who are 'the name' in the power supply business, retails at around £45.
Thermaltake are also making a name for themselves in this area -
this one is in fact the one I'm running and it retails at around £35.
As an indication of what to expect from it, that 360W is powering an XP2000+, 512Mb of DDR ram, 2 7200RPM harddrives, a GeForce 3, several other PCI boards, and 3 fans and is having no problem (you can tell by looking at the voltages with a utility such as MBM5).
You can get units rated higher in terms of watts for less cash, but be aware that at the cheap end of the scale you will be getting a powersupply that almost certainly will not reach anywhere near what it claims to be able to provide (ie. you'd be lucky to get 320W out of a 420W cheapo one and have any stability) and will be prone to voltage fluctuations under heavier loads which can seriously destabalise the system. I've seen the problems of several people I know solved by buying a decent PSU because they were trying to run large ammounts of kit on a cheap 300W power supply and their systems would fall over as soon as you did anything to drain more power (eg. CD burning).
It is worth investing in a quality unit.