Here is a site where you can check the sales of anime DVD/Bluray.
http://dvdbd.wiki.fc2.com/
It's in Japanese, but you might be able to check it with google translate.
And here is the sales of DxD:
http://dvdbd.wiki.fc2.com/wiki/%E3%8...%83%ABD%C3%97D
DxD sold 10000 copies average per disc. In terms of sales it did the one of the best out of all the anime in that season. So it would be natural for the anime production team to make a season 2 if they know it will sell like mad again.