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Pedantic Grognard
Well, you remember what you remember, but dragging out my copies of AD&D 1st edition books and looking at the covers:$50 US in 2020 is about ~$15 in 1980 based on online inflation calculators ($1 in 1980 is about $3.30 in 2020). I remember the books costing closer to $25 in the early 80s, which is closer to ~$80 each in 2020.
Player's Handbook, price printed on the back cover, lower right corner: $15.00.
Unearthed Arcana, price printed on the back cover, lower left corner: $15.00.
Wilderness Survival Guide, price printed on the back cover, lower right corner: $15.00.
Dungeoneer's Survival Guide, price printed on the back cover, lower right corner: $15.00. (Child World/Children's Palace price sticker on front, $11.97)
Greyhawk Adventures, price printed on the back cover, lower right corner: $15.00.
Manual of the Planes, price printed on the back cover, lower right corner: $15.00.
My copies of Oriental Adventures, the Dungeon Masters Guide, the Monster Manual, the Monster Manual II, and the Fiend Folio do not have prices clearly printed on them, but they have some sort of TSR product number on the back cover near the ISBN that begins "394-" and ends either "TSR1500" or "TSR1200", which I believe were indicators of the MSRP.