If you look at the details, Apple is now selling "cheap" and "expensive" iMacs. The Retina iMac doesn't only have a retina display, it also has faster CPUs and a better drive in the base configuration. You pay $2499 for the base Retina iMac; you pay $2199 for the same hardware without retina, 100 MHz less clock speed, and a lesser graphics card, so the actual price for the retina upgrade is less than $300.
I don't think you should expect an upgrade for the non-retina iMac any time soon, and not without an equivalent upgrade to the retina iMac. Non-retina will always be less powerful hardware, not just a cheaper display.