Well the big problem with getting used Macs is two fold.
First, Supply.
They don't make a whole lot of Macs in the first place.. not compared to the wide variety of PC's out there.. I mean, if you want a PC you can get a Dell, HP, Compaq, Gateway, etc.. but if you want a Mac.. well.. you pretty much have to get a Mac.
Second, Demand.
Historically, Macs are useful for a very long time. For instance, a G4 400Mhz Mac that was made going on 8 years ago can run OSX 10.3.9 with no problems, and while apps like PhotoShop CS might be a little taxing, for general purpose eMail, web browsing, iTunes, stuff like that.. it's really quite peppy and capable.. not to mention aftermarket upgrades are available to take those old G4 towers into the 21st century with dual 1.6Ghz CPU's.
Because the computers are so viable for years after purchase, people are far less likely to upgrade as often as your typical PC user.
High demand + relatively low supply = high prices
I've watched old iBooks with dead logic boards and busted screens sell for CRAZY money. Nothing about a Mac is inexpensive.. new or used.