Forgot to mention that I agree with the thesis above, that the Cubs would not be thinking about renovations of that scale if they did not think it would pay off for them in the end.
At some point fairly soon, they probably have to do something substantial and structural with WF to make it safe for years to come. And at that point, you might as well go whole hog and add luxury boxes, which are the probably the best option for generating new revenues inside the stadium (seating seems pretty close to being maxed out). All the retail stuff can help, but that's also an 81-day proposition (I imagine they will find some more uses for the stadium during non-baseball periods).
I have my doubts about the $250M price tag, because we all know that construction cost overruns are pretty much standard (I don't recall what the original price tag for BIII was). Keep in mind that the Soldier Field renovation cost a fortune, and I don't think the circumstances are too dissimilar with WF if you are talking about rebuilding the entire upper level, which would likely be the case with luxury suites.