With that kind of money you could make three huge blockbusters. I think I'd do something like this:
- Reboot Batman movie that has a characterization and film style that lends itself more to a team-up (you can see where this is going).
- Reboot Superman movie. Something that does not involve his origins but is not too far into his career either.
- Superman/Batman movie. It'd have the appeal of a team-up movie like The Avengers but much simpler than doing something like Justice League. Maybe throw in a cameo for Wonder Woman, Flash, Green Lantern, or Aquaman if you want tease a future Justice League movie (or standalone movies for any of those mentioned).
I guess the first two steps aren't much of a surprise; that's probably how anyone would start. But I'm of the opinion that a Justice League movie is too difficult to do. It's hard to put my finger on it but it seems like The Avengers was possible because it's easier to write those characters into a live-action movie since most of their powers simply rely on hitting and shooting. The powerset of the Justice League members is so diverse and unique that it'd really take alot of work to write something that's not really lame. But that's just my opinion; maybe someone will prove me wrong one day.