What is the cheapest '51. In my opinion that is not the question. The question should be what is the cheapest way to accomplish a design.
Case in point:
A basic '51, 8k of external RAM, a small CPLD to provide I/O and the ALE latch.
OR
A 89c668 with all 32 pins free for I/O since with internal 8k RAM no external memory was needed.
While the '51 in case 2 is more expensive, the solution is cheaper, not to mention the saved board space.

Erik

Erik