AMD 3000+ Processor - Socket A? If so, worth about £50.00 max second hand.
What motherboard? Unknown motherboard for socket A - max £30.00
80Gb Hard Disk - about £25.00
512Mb RAM - What make? If generic - £30.00 tops.
Graphics card is fair - about £50.00
DVD-ROM drive - £5.00
56K Modem - £5.00
15" Monitor - you couldn't give one away, but let's be charitable and say a tenner
Windows XP Home second hand, what, £40.00?
Lexmark all in one, not worth buying second hand, but OK, we'll say £20.00
And for the case/PSU; floppy drive; speakers, mouse & keyboard - second hand, about £50.00, unless they're really really good.
What does that add up to?
Hmmm..... I make it £265.00.
Now, I must point out that I don't know what a lot of the parts are or if it's a socket A or 64 bit system. And I've been harsh on the all in one printer as I think Lexmark are rubbish. But from what I can work out, £270 tops would be a fair price.