So, did you really use a "real time" example with a real ship? If so, what was the cost of building the ship? Given that, a person could come up with a scale on how many months a person had to have it in service (at the asteroid) at a certain percentage in order for the ship to "pay for itself" and begin "sending back" a profit.
(yeah, I know, I know...git off yer back, right?
)