I suspect that the entry level Retina pro is only as "cheap" as it is because Apple is assure of capturing all the upgrade revenue. Further, if they were offer

I suspect that the entry level Retina pro is only as "cheap" as it is because Apple is assure of capturing all the upgrade revenue. Further, if they were offering an upgradable model in addition to the current retina option, in addition to the existing model, they'd both be more expensive than if only one was offered, because the screen supply is constrained, and with the market split, the volumes on the thin/light model would be lower.