Do you have a source for that? I ask because that was the one price I couldn't get a definitive answer on - I can find the $12 price for a couple of years later, but nothing from 1978 itself. (The adverts in The Dragon didn't give a price.) So, being able to put an actual source to that number would be great.
Unfortunately I don't have a definitive source either, just hearsay and talk, but that $12 number seems pretty consistent across the board where I've looked, and I've yet to see any different number.
I've read $9 for the Monster Manual, $12 for the PHB, and $18 for the DMG during first print runs. Even if we want to play it safe and assume $9 for the PHB, that's still about $33 today, for a 128pp book, which I'd call "high" on the price spectrum throughout D&D history for that amount of content, so I think the general point of my post is still accurate.