You'll have a hard time finding any $70 cash rent around here. Since you apparently like to use the extreme numbers mentioned, put the $30 in and see how that turns out.   I think you're $30 for added costs is a little optimistic, too, judging from what I see corn belt guys talking about in the way of N and herbicide use. 

I'm guessing that Matt's yields would be more consistant then ours, that's really the only basis for my statement that he may have a better situation then we do. The reason for that guess is that he is reporting decent yields with very limited (9") rainfall. This tells me he must have more field capacity then we do. We are dependant on timely rain to make a crop.