I have been aware of the following issue for some time, but I hadn't noticed any entries in the spreadsheet mentioning price discrepancies, so declined to post about it in this thread, thinking price differences were perhaps just outside the scope of this project. Browsing it again today, though, I did see a couple such mentions, so here goes.
The following Talisman: Digital Edition DLC have been $1 cheaper on Steam (at least in the U.S. region) for a long time now:
The Ancient Beasts, The Cataclysm, The City, The Clockwork Kingdom, The Dungeon, The Highland, The Realm of Souls & The Woodland (all expansions): each $4.99 on Steam vs. $5.99 on GOG (20% more expensive here)
Complete Runestone Deck: $2.99 on Steam vs. $3.99 on GOG (1/3 more expensive here)
(Meanwhile, GOG's DLC Bundle is priced the same as Steam's content-identical Season Pass, even though they include the majority of the aforementioned price-discrepant DLC.)
Additionally, a standalone single-player game, Talisman: Origins, is also a dollar cheaper on Steam -- $5.99, vs. GOG's $6.99 (1/6 more expensive here). (However, its three DLC expansions are all consistently priced between the two stores.)