IIRC, the IIGS was sold from 1986 right through 1993 (although production stopped in 1992), and it used a highly optimized production line, so was able to crank out a lot of units. The IIc used the same style of production, but only ran from 1984 to 1988 -- and was competing directly against both the IIe and the Macintosh (as well as the IIgs for two years) -- so while it was cheaper to make and buy, it had a more limited market.
I know in the education market, the big sellers were the II Plus, the IIe and the IIgs, before things went crazy in the Mac years. I'm pretty sure it's the education market that drove the numbers for the Plus and GS, and the e just hit that sweet spot, as production optimized and ramped up, education AND home computing opened up, global markets became available, all at the same time -- AND the e could run all the software that ran on the earlier Apple computers. Plus, it had stability, having a product run of 10 years. The Plus essentially paved the way and provided funding for producing the e -- the e was a marketing and production continuation of the Plus.