It wasn't only 1972. The price difference also appears for the 1971 model year:
http://www.oldcarbrochures.com/stati...cs/image8.html
Why?
I can't seem to find a definitive answer for it. I will take a wild guess though and say that the camshaft in the "without power brakes" is a little wilder then the one with power brakes. Maybe Olds outsourced the cam from a 3rd party and passed the added cost on to the buyer.