There are various tiers, but anything from $25 and up at least gets you a digital copy (Steam or GOG; your choice) of the game. There are tiers both physical and digital above that - boxed copies, extra downloads, collectors edition, a roiling excess of maps, t-shirts, soundtracks, strategy guides, artbooks, and other shit.
The $1000+ rewards are a bit... different, in that they let you design an item or an enemy encounter or an inn or shit like that.
I for one look forward to picking out which character portraits are based on nerds with way too much money.