SpinTires: MudRunner releases October 31st on PC, PS4, Xbox One.
50% off SpinTires: MudRunner PC version if you already own or purchase the original SpinTires before October 31st.
cdkeys.com is out of stock for SpinTires $8
steam has original spin tires for $29.99 - not a deal
eBay has several sellers that are selling digital code for original spin tires $10 - $22.
If you can buy original version before October 31st and add original version to steam, then new version SpinTires MudRunner costs $14.99 or $13.49 (Half off current steam price)
50% off promotion is only good for PC version. PC version is only version that supports adding mods in the game.
Original version has great reviews on steam.
eBay Listings for US sellers of original spin tires:
Developer website, forums, FAQ, etc.