Okay. I haven't got time to give you all the evidence that will convince you, but just as quick study, his a link for Macintosh G5 tower. It is £390. It has been obsolete for about 6 years now - the chip is PPC and so won't run new software. It is 2Ghz dual core but only has 1GB of RAM preinstalled.
http://www.studiomacs.co.uk/secondhand_mac_g5_2.0DP.htmlAssuming this was put out in 2006 (the latest it could possibly have been put out), it's actually very difficult to find a PC of similar spec and age on sale. However, I found something comparable on ebay, probably from around 2006/2007, with considerably better spec, going for less than £100
http://www.ebay.co.uk/itm/HP-XW4600-PC-Intel-Core-Duo-E7200-2-53GHz-4MHz-Ram-160HD-Monitor-if-reqd-/150902001053?pt=UK_Computing_DesktopPCs&hash=item232275cd9dI know this isn't conclusive, I'm just trying to give you an idea. Macs really do retain their value much longer than PCs. This isn't necessarily because they are better, it just very much is the case that they hold their value much longer. Any amount of time spent comparing makes this self-evident.