yes that sounds logic, its more economic to buy a new iMac every year rather then investing in your existing one ..
what happens if people on ebay realize that they only get unwarranted generic hardware in a fancy box and that a iMac can fail like any other pc form dell , hp, sony...and the prices would drop to a realistic level for a used computer
and osx alone does not change anything with a snow leopard value of $29 it can be neglected in the overall value
in the old day's of ppc that was different like the slogan of apple "think different " back then you got not only the fancy box , but hardware that was unique and never seen in a pc again after i think it was windows 3.1 that could run on ppc , and please dont start with the $1000 display you get , that cost from apple as replacement $600 and apple is likely to make even a profit on the replacement parts so my guess apple get these displays for around $400 if not for less
and yes you can buy a equal hardware and put it in a generic box , it might not come cheaper too , but only because you calculate with retail values on the parts you buy , take the profit of the retailer away and you can build the box for a lot less ...ok not a all in one , but something that looks very similar to a MacPro
PearC springs into my mind
http://news.softpedia.com/news/PearC-Launches-039-the-PC-with-Mac-OS-X-039-103953.shtml
http://www.pearc.de/en so you could get the power of a iMac i3 for the price of a Mac mini and within the difference of about €500 you will get a nice display i suppose , but the best bit is generic parts inside , so you can replace parts with whatever you find (within limits of being osx compatible) so replacing parts wont break the bank that normally render a iMac beyond economic repair once out of warranty , and more importantly it is fully user upgradable