Quite a bit, even if just current WoW subscribers goes to the movie, that's 8 million x 13$ box office ticket. That's ~104,000,000 in guaranteed box office revenue.
On top of that, people are going to drag their families to these movies, not to mention the regular joes and random people who decide that Warcraft is the movie they are going to watch that week.
It probably won't be a billion dollar blockbuster like Avatar or Dark Knight but I'm sure it won't be a total flop.