The big problem with downloadable content in multiplayer games is the split between the haves and the have-nots, with those who don't want to shell out more money for extra maps being left in the dust to a certain extent. That's not going to happen with Turtle Rock's Evolve, because all its DLC maps will be free for everyone. (Well, everyone who owns the game, but that was probably a given.) Creative director Phil Robb confirmed as much to IGN, stating that “We can’t break the community up, so all of our maps will be free. We never want anyone to get booted off of a server because they haven’t bought something".
While players will be getting the maps for free, they will have to hand over a wadge of their local currency in exchange for additional characters and monsters. The good news, however, is that you'll be able to play alongside these new hunters and creatures, even if you don't own them yourself. Here's Robb again explaining that.
"We'll have characters and monsters, and even if you don't buy them, if your buddy wants to buy the DLC, that's cool, and your game will be enhanced through his purchase. So, you won't be able to play as those characters, but you will certainly be able to play with those characters."
Which is an entirely sensible way to handle DLC in a multiplayer FPS. Evolve is out in February, and your PC will need to be this tall in order to play it.