This isn't a theoretical argument. I actually did this and it didn't take all that long and some of it was even fun. An added benefit is my specs run faster, too. So developer time is saved on every spec run now. You also hit that intangible of improved developer happiness.

Additionally, the X time exceeds Y cost argument really only works when people are optimally efficient. Clearly those of us posting HN comments have holes in our schedules that might be able to be filled with something else.