Interesting thought, it's been tossed around forever, but I doubt we'll see it happen. Besides - who throws out their old mac?! Macs hold their resale value quite well. I eBay all of my old machines. My 24" auction is set to end in 51 minutes. And I always make up at least 75% of the cost of my new machine.We assume the tower would sell for less money because it does not include an LCD. Also we assume the tower would have an upgradeable video card and have slots for more RAM. It could also hold more internal disk drives so as not to cluter the workspace with little boxes and cables and wall worts power supplies.
Two problems with the iMac is that when the computer needs to be replaced you have to toss out a nice LCD monitor with it. and there is no "video in" plug on the iMac, so you might need another LCD.
Ten years ago Apple was able to sell what was then a high end Power Mac inside a tower case for $1,500.