This doesn't sound like premium DLC, so I'm a bit worried also.
If Warner Bros don't think they can charge $15/£10 for it then it might be on the small side, like those DLCs.This is the publisher that brought us Evolve, after all.
Evolve is a 2K game, yo! But I do agree with your point; $5 DLC seems to raise a few flags. I would really love to see a $15/$20 piece equivalent to Harley Quinn's Revenge for Arkham City. That was a great piece of DLC. Granted, I've already bought the Season Pass, but I hope to see something big come out of it.