In the case of the OP, who does not print, why bother with monitor calibration at all? The 27" iMac Retina Display is beautiful right out of the box. And in this case, the issue is purely subjective. Whether his monitor shows the proper color of an orange or apple when viewed under controlled lighting situations is irrelevant right? There is no product line requiring color correctness, etc.
My advice is to do the basic software calibration provided in OS X and leave it at that. No need to spend any money at all.