I don't think it would cost more than the regular CGI-filled flick, or more than a Dreamworks movie. It could be done if they wanted to. But I'll agree that not as a direct to DVD, no.
I still think a Justice League could only be achieved with full CGI, I'll leave it there.
I think the Avengers has proved that wrong...
That's the thing, to me Avengers and Justice League are different kinds of beasts.
Avengers took 5 years of good but predictable solo movies to build up. If Warner tries to do that again with JL, they're going to fail inevitably. If they want to do it, it must be all in one film.
But erasing those years of building up the characters takes away the cohesion Avengers had. We were able to believe all those guys could be in the same universe fighting together because they made us believe it through the solo films.
Also, I've always thought Justice League members work better alone, because of how different they are from each other, because of their adventures and the worlds they belong to, because of how different they costumes are, etc. I don't know if I'm making any sense, but to me, the only way to gain the cohesion the Avengers film had in a JL feature film, is have it full CGI. All characters animated with the same tecniques will make them gain that cohesion, and will make clear for the public that they are staying away from any continuity the other DC films had.