I think Valve/Steam will refund you the difference if you request it
Yup, in steam support you can say you have a problem with a game and one of the options is “it is now available for cheaper” and they’ll give you the difference if you bought it just before a sale.
Yeah you can even just refund the game and they usually fulfill it in a day or two, usually in time for the sale.
My solution, refuse to buy until it’s on sale. Got plenty to keep me busy 😃
This happened to me with Middle-Earth: Shadow of Mordor the year it released. I got tired of waiting for it to go on sale, so I finally bought it at full $60 price. The very next day, it was on sale for $5.99. D’oh!
LPT: do a quick check for the price history of basically anything you buy, especially easy with Steam games by going to https://steamdb.info/
My solution to this is to buy at a price I think the game is worth for me, and if I overpay, I think of it as supporting the developer to make more like it.