To put this whole issue simply- is it possible to use an isolation shield as a (maybe temporary) 'replacement' for room treatment? I mean I realize that nothing will replace a good room, but in a pinch, or for those who can't afford it, is it possible to opt for maybe Monoprice's really cheap iso shield and save $750?

I just found this when I googled Iso Shield. http://www.iso-shieldacoustics.com/products.html  What a great idea, I'm going to elaborate on this concept and build one with rigid fiberglass. As to your question: I.M.H.O. You can definitely use solutions like this for tracking but not so much for mixing. There are lots of budget do-it-yourself room acoustic solutions out there but if that's not feasible the VRM box by Focusrite is  pretty amazing if you are without a decent room for mixing.
 
Rocky

Definitely go the DIY route. It'll save you money, and you won't be nearly as disappointed.

darn…. diy doesn't seem to look quite a clean to me! plus it's probably only a $20 saving, plus all the labor. I got a pop filter for Christmas, it's FAR higher quality than the one I was planning to build, and apparently cost only about $10 more than ym production costs as well.. What is truly a better choice between DIY and buying a cheap one? (iso shield, that is)

one person's experience...
 
I'm not a big fan of the mini isolation shields. I don't like the resulting sound quality. I have spent some time with two different models, from two different manufacturers, and a third that I put together for myself with a music stand and some spare Sonex panels (yeah, really ugly!!!)

For voice-over work any of the three is ok for most voices, and with really careful microphone selection, and placement, the right voice, and a little bit of luck they can be very good.

For everything else I tried (Acoustic guitars, hand percussion, solo strings, brass, and winds) I was disappointed with the results. I get much better - to my musical tastes - results using plain old gobos. Yeah, they are large, and a pain to place, but they just work better for me.

I really hope that someone, someday, makes me eat my words!

those shields can help, but still, it is when you do it and external noise to cut down on.
Early in the morning for me or at night late late towards morning.