The Cortex A series cores are around 2mmx2mm. The Cortex M series are something like .2mmx.2mm. It looks like my 10mmx10mm comment was way off.
So, let's assume that the 8-bit MCU's are .1mmx.1mm. That's probably WAY too big, but let's start somewhere. That means that you can fit 7,000,000 on a wafer. For roughly 3,500 wafers for the entire, worldwide capacity of embedded MCU's.
The entire, worldwide, annual market for MCU's is 3,500 silicon wafers per year. That's the same number of wafer starts that a big fab has IN A WEEK.
And that was 2008. http://www.eetimes.com/document.asp?doc_id=1169357
Modern fabs can pump out a million a week: electroiq.com/blog/2014/01/tsmc-samsung-and-micron-top-list-of-ic-industry-capacity-leaders/
This is why everybody wants to produce bigger chips like SOC's and why Intel will never switch to ARM until it's about to go under. If your chip is too small, you can't move enough to fill your fab even if you give them away free.