I've always found this sound, rational ROI driven approach to product management a little off the mark. Software isn't like real estate or investing in TBills - you don't invest $X in development and get a nice 10% annualized return on investment over the next 10 years or something like that despite how seductive such thinking can be.

It is largely a "hits" business where 1% of the activities that you do result in 99% of the revenues. The returns are non-linear so there should be almost no focus on the input estimation. If your feature only makes sense if it can be done in 3 months but doesn't make economic sense if it takes > 6 months - delete feature.