During my current (first) playthrough, at one stage I sacked a couple of brainers because 1) I could barely afford to keep them. 2) my research was just about to hit a bottle-neck. (I think I had around 13 at the time, and reduced it to 11.)
Not long after that, I finally captured an academy engineer, and the research tree opened up like a flower. Fortunately my financial situation improved a lot as well. I pretty quickly expanded my research team to 18, then started a new research base... Now I'm close to the end of the game, and I've been going along comfortably with ~29 brainers for awhile. I certainly won't need any more, because I'm running out of research topics.
In any case, I do think the high cost of brainers is a bit of a trap. But I don't think it should needs to be changed (I don't mind either way). If a player runs out of money, they can sack their brainers. No big deal.