The weird thing is, Green Lantern would have been better if they'd taken the source material more seriously and did it as an epic Space Opera rather than a traditional SH flick, while Flash won't work unless they play up some of the campiness of it and poke fun at some of the tropes it invokes.
Looking at Flash's villains, most of them are at least a little outrageous in terms of their costumes and powers. That's probably part of why Wally was so great as the modern-day Flash, because he was capable of mocking and drawing attention to the goofiness of some of the stuff that happens in the pages of Flash comics. He's a funny superhero, and I have long contended that he is indeed the DC universe's equivalent of Spider-Man.
I would hope that if they do another Green Lantern movie they try to feature the sci-fi aspect of it more prominently and take the story a bit more seriously, and that if they do a Flash movie they let it be what it is-- a fun Superhero movie that isn't afraid to poke fun at genre conventions. It would be a sad day if they took Green Lantern and turned it into a movie series that can't take itself seriously, and then made a Flash series where Barry Allen is his usual boring plank self and it tries so hard to be serious that it sucks the fun out of the source material.
Also, I have no idea how much Green Lantern cost to make-- I've heard figures anywhere from $150 million to ones as outlandish as $300 million. There is no way Flash would need a budget of 200 million or more unless they were SERIOUSLY overproducing it. As has been stated, super speed is not an expensive effect to achieve on film, and a good Flash movie could be done on a budget of around 100 million. If Weather Wizard is the main villain then you will need a budget for some good disaster effects which would probably place the budget closer to 150 million, but even that is not too big for a summer blockbuster by today's standards.
Flash does not have to be the next "The Dark Knight." Greg Berlanti's treatment was described as "Silence of the Lambs meets The Matrix," and that bothered me immensely. I'm not sure if DCE will be keeping Berlanti around after the heavy criticisms the script for Green Lantern has gotten, and at any rate, I am worried that he has the tone of Green Lantern and Flash backwards. Green Lantern should be serious, and Flash should be funny, and yet from the sounds of it we are headed towards getting it the other way around. I someone at WB got excited about him before thy really had a good sense of what he was going to bring to their comic adaptations, and I don't think it was a smart idea to attach him to Flash before seeing what kind of reactions Green Lantern gets.