Well, La Jolla Renter wanted to keep things simple, hence the 29 year mortgage and the constant mortgage rate. One could do a much more sophisticated calculation and probably arrive at the same ballpark figure in the end. I think simple is good.

However, my comment would be that, although the example above looks good in theory, in practice couple B would probably just spend their extra $600/month on useless junk, thus invalidating the whole theory. Pretty cynical view, I know.