Bar soap!
When I was little & my mother was sick I got sent around to various relatives to live. A lot of the old folks used bar soap shampoo. Some of it was nasty, some was nice-smelling. It worked fine.
Then when I was older & a musician I used to visit people on their communes - hippie junk. Their bar shampoo was never as nice as the good-smelling stuff from my childhood.
Anyway, I don't care what I use as long as it gets the hair clean - but I use the really cheap shampoo from the discount store (you know, the 33 cents a bottle stuff) and it's perfectly good.
My son who is allergic to everything has to use this special stuff with a prescription, so I figure it balances out financially.