Say you wanted to build some kind of multi-computer server farm / or just wanted to buy some more pcs / or just wanted to put together a super cheap pc / or just wanted to upgrade the core of your current pc (mat?), here's some options, and prices inc vat:

Intel Pentium 4 / Celeron System: (Socket 478)
PCCHIPS Socket 478 motherboard: £25
Celeron 2.26ghz: £44
Total: £69

AMD 64 / Sempron System: (Socket 754)
PCCHIPS Socket 754 mobo: £32
Sempron 2600 (@1.83ghz): £43
Total: £75

AMD Athlon XP / Duron / Sempron System: (Socket A)
PCCHIPS Socket A mobo: £15
Duron 1600: £27 (/a Duron 1800: £29)
Total: £42 (£44)

The Socket A (AMD) platform clearly offers the cheapest system - and I suspect it'll be quite some time before anything challenges it on price... (just add 512mb DDR ram for £30, and a heatsink/fan (if not included with the cpu) and you're set) ... if you wanted something even cheaper, you'd probably have to look on ebay*

*although you could end up paying more or the same for something worse such as this Duron "1800+" - whereby the motherboard manufacturer (ECS/Elite/PCChips) have decided to rip people off by giving something like a Duron 1.3ghz chip a "performance rating"... which is totally unsanctioned by AMD (Duron's have never had performance ratings applied to them).