I'm in my 50s and I don't ever recall a time when B&W was cheaper than color for amateur use. To get B&W movie film was damned near impossible, in fact. I think at 16mm level B&W was cheaper, maybe.
No idea about 35mm, but from personal experience, 16mm black and white stock is half the price of 16mm color stock. The processing cost was the same regardless of color.