Good crack, but I think you've messed up a little
Leaving the rest of your assumptions is as, you would need to do $8000-$400 = $7,600, and per day not per week, because after 1 day the machine moves onto another site and saves the build costs there there (not after 1 week). So payback would be 223 days or roughly one working year ($2m / 7.6k = days)
No consideration of all the other savings like saved interest on property and earlier returns for the owners which would pass through the value chain, reduced waste costs, reduced insurances, reduced labour oncosts, etc. (offset partly by maintenance and operating costs, as you point out)
