I don't know the actual numbers but the micro S has been known for a long time. Obviously, there are less of the micro S than the regular S but not too much less. I think the price is also higher because people look for these coins. They are not hard to find at all.
In "The Complete Guide to
Mercury dimes Second Edition", the author references another person who handled over 160,000 dimes over a period of 7 years. It was reported that 776 dimes were the regular S and only 75 dimes were the micro S. This is about 10% of the total.