Last year I went to the Bears @ Cowboys MNF game so I could see the new behemoth of a stadium (aka "The Deathstar") and I was able to snag two tickets day of for face value. That place, in my opinion, was about 65-70% Bears fans. It was really sad to see.
With that said, I've been to games there in the past vs the Broncos and as Bacchus stated above there are a lot of nearby Broncos fans. I'm not sure you'd have the same luck buying tickets day of through the Cowboys website. I'll probably be flying in (If I haven't relocated back to there by then) and I wouldn't risk that. Im rambling.. I'll stop.