I see a few DLC scenarios:
- I'll take it like Skullgirls-If the DLC happens to be based on community choice (IE: Polls and request petitions decide it) then I'd say have it be free for a couple of weeks to a month for people who are actively playing and supporting, and then make it paid for those who aren't.
- Otherwise, if it's pre-decided, I'd have it be always paid. That's content that they worked hard to get out post-release and at that point I'd rather see them get paid higher to make profiting content and successful sales thanks to DLC.
- Third option is to have a "bundle" deal and have characters be 50/50 predetermined and requested at a reduced price together for people who really want the additional toppings. Again, cake metaphor.