I'm not sure what I typed in, exactly, because my computer won't tell me right now, but I think I just typed in something like
what did encyclopedia set cost in 70s
If you do that, the thread I linked comes up as #4.
I tried a lot of other searches specific to Britannica, but only found people asking what an old set of Britannica is worth now, etc. I did find a sort of news article about a guy who collects and still uses encyclopedia sets. He said his parents bought Britannica in the 70s from an encyclopedia salesman and it used to come (one by one, I assume?) in boxes and it was a big deal, opening them. But he didn't say how much they paid.
If you want, I can find that story for you.
I also read the description of a 70s set on Ebay and it sounds like with the leather binding and gilt edges and color illustrations, they were worth the money!
I vaguely remember my mother picking up the new Funk & Wagnalls volumes when they came out. She bought them at the A&P. I think they only cost a couple of dollars each, at the most. No gilt.