For a change
Pretty much all of the comment below is more informed than usual, and certainly the politicians. I would bet that there are some old contracts where a PC is expensive, but when you consider what happens to a basic PC, assuming a one off, it's not that surprising, TCO costs come out to £3500.
1. Raise order on IT provider
2. IT Provider orders PC from its supplier.
3. PC delivered to central build facility
4a. Build facility discovers Dell have changed the spec and a new build is required so the PC is built by hand and then imaged for the next.
4b. PC is built with standard image.
5. PC is sent from central build to site IT, which could be the other end of the country, but you only charge the same.
6. On site IT install the PC check it works and take away the old PC
7. Old PC cent to central IT where hard disk and memory are removed, Carcass sent for recycling, hard disk and memory are wiped, and either reused or securely disposed of.
Note that a good many PCs in government are now delivered with encrypted hard drives which has to be done after the machine is built and takes several hours. They may also have removable hard drives. The PC cost may also include all the network infrastructure costs on the site, and the WAN. The WAN may well require expensive Crypto units, or even a dedicated network, which again adds to the cost. This happens because some departments like a unitary, per user cost, and if you have offices all over the place supplier have to factor in deliveries from Shetland to the Scillies.
Oh, and most of the government contracts I have worked on have open book accounting with fixed margins which are currently running at 3 - 5% for this kind of stuff, so the PC itself is usually cheaper than you could buy in Tesco, it's the configuration & support that pushes the cost up.
A better comparison would be £500 PC/Monitor + Windows 7 Pro + Office Std + Home Router + 3 Years Broadband + Extended Warranty + Home Service from the techguys (TM).