4x8GB of RAM sticks for an iMac could cost you as little as ~$300. OWC 32GB of RAM set costs ~$400. I'm not saying memory isn't a good reason to move to a Mac Pro, or that you aren't right that in general using 32GB of RAM with an iMac isn't really a great option.
Last time I priced them, they were a bit more.
Running a MP over a iMac is actually a combination of several things.
1. Memory - I need more than 16 gb. I needed it long before it was available on the iMac.
2. Heat dissipation - I have way too many Apple products that died because Idiot Boy was more concerned about looks than heat dissipation - Apple TV, Time Capsule & worst of all a $1500 MBP that died before my $300 netbook.
3. Screens - the quality of the iMac is variable.
4. Internal storage - I have 6 internal hard drives - and I could add 4 more. The price of an iMac & a thunderbolt enclosure is about the same as a MacPro.
5. Speed - If you are into these things, MacTracker shows a 1,1 having a 4,388 average geekbench score - mine is much higher (6,257). If I spend $140 for 2 Xeon 5355's, I am running about as fast as a iMac 12,2. More bang for buck.
6. Repair speed. If I want to add an SSD to a MP, I just pull a sled. putting an SSD in an iMac is a whole different ballgame. If the video card dies, or I just want a new one, it is easy to replace (Nvidia 7300 [DOA] - ATI 3870 [fan died] - ATI 5770 in my case)
7. TCO costs. An iMac will put you on a 36 month replacement cycle. I am looking at a 72 month replacement cycle. I can't move memory into my next MP, but I can move the drives & the video card.