That's approximately my recollection. I specifically mentioned the PDP-11/20 because that is the one I recall as being "earliest." My memory could have been wrong on that point, and you have clarifed the details some in my own mind. Thanks.
I personally paid about $300 each for 4k dynamic ram cards from MITS that didn't even work right because they were designed wrong. That was my first, and forced, serious foray into the wonderful world of electronic design. I fixed them. But it took a month of learning and doing. That was... I think end of 1975, maybe early 1976. Not quite $1/byte, but not cheap and it came out of my meager (very meager) pocketbook.
Jon