Part of the GOG.com WE LOVE GAMES SALE - see more deals here: https://www.gog.com/promo/we_love_games_sale
Stardew Valley equal all time low price when checked on Steam history (same in Dec 2019). Note this does not give you a Steam key. Instead it gives a DRM free version of the game (No activation or online connection required to play.)
Also money back guarantee. 30 days coverage after purchase.
Just paid $11 on Steam a couple of weeks ago (https://www.ozbargain.com.au/node/514860). Great game. Get into it.
But little sad I did this before the Animal Crossing release… barely enough time for one time-sink life simulator let alone two.