It currently costs 19.99 on the steam marketplace, and they have extra character packs you can buy for $2 a piece (which I recommend because the nightfall extra characters are soooo badass)
Since it is a steam game, and steam leaves all of their games "open" as in, people are allowed to mod the levels, stats, gameplay however they want as long as they're playing on the steam servers.
Some fun features to mention:
Welding doors shut to detour zombies and strategical attacks
Perks (CoD style more or less)
Slow motion (you have to pull off a lot of headshot kills in a row and it'll go into slow motion)
In all seriousness people, hold off on buying your next longshot you plan to