This NEEDS to be free the first time around if they want people to stick with the game, and try it out.
If they gain the reputation of valve of alot of post-release support without having to pay, it'll get alot of popularity.
At the very least it should get 2-4 new maps, and outfits/Guns for free initially. Maybe a game mode
Splash Damage would also acquire Valve's reputation for ruining great game balance for gimmickry.
The first DLC should be free to counter the unjust review scores. Also, of course BRINK will have DLC at some point. Be patient, be aware.