By the time you add an SSD and 8GB of RAM to a brand new cMBP to bring it up to the specs of a similarly-specced rMBP, it's already going to be just about $100-200 below that of the rMBP, and you're still stuck with the horrible non-retina display, mediocre iGPU, thick chassis and a heavier weight.
Yes but this is life, not Top Trumps where a few grams of weight are necessarily soooo important. My MBP has been upgraded with drives that didn't exist when it was built so a price comparison then was not possible. Add in phasing the expense, and the fact that the drives can be transferred to another suitable machine so they are not lost if the MBP dies in any case...and $200 is still $200...