I have often thought about this as well as I work in construction and use cordless tools on a daily basis and the dealer msrp on batteries is gross. I
I run 5 cordless tools and have 6 batteries on the go daily and this is just me alone.
Trouble is the batteries last about 6-8 years and are the strongest part of the kit, I have 3 batteries that are 10 years old and still work great but I have gone through 3 chargers and several tools.
If you compare the price of the batteries by themselves which for my favorite brand lets assume $100 for one 3Ah OEM battery on sale and you look at your favorite tool dealer, I find that they will sell a drill with two batteries, a charger, a case and sometimes throw in a 3rd battery for free: they will sell the kit for approx $250. (Canadian dollars) I have to go with the complete kit for sure or I will end up buying a "tool only" for another $100. It would be pretty hard to make and sell batteries for less than $80. Now if you are a handyman then for sure buying a battery because it ran flat and died from not using it. You would be the target market as you will probably never wear the tool out. Just my opinion.