Opportunity cost in the analysis is generally hogwash as Maistre Gracey said, at least in terms of its explicit inclusion. Just because one does things in a spreadsheet does not make things more correct or more accurate. I know this bugs purists who want to do things perfectly "right" but the intractability of the "right" calculations for the average person makes the quick and dirty method not only easier but not appreciably worse in doing your calculations (and certainly not different from a qualitative decisionmaking perspective).
Still don't agree with excluding it, but from one former NIU student (and 9 month DeKalb resident

Oh what I'd do right now to have some Lukolos............
Chris