Look at the price of a stand alone comparable Dell display and you will see the Retina iMac is a bargain, for virtually the same price you are getting a complete computer.
No. 5K displays are a very new technology (the first ones were released just last fall) and as such the "5K super expensive one" is actually reasonably priced. The 21.5" does not have a Retina 4K display option yet although one may not be far off.