That is looking at it from one angle, but buying is only half the story.
For an operator to have 30 or more BC-17s flying around, they will have to be doing more of their work outside the USA. Those aircraft can't be completely used up flying around America to the oil fields, so at least some will need to be dropped into the Gulf or Africa doing non-US charter work.
If there are other operators, like Volga-Dneipr, operating internationally with fleets of reworked Illys then that dilutes the already restricted amount of work available to a civil heavylifter. The technical side of the BC-17 and acquisition is one thing, finding work for them is another. There may not be anything like enough work for a US-registered and expensive fleet of Boeings.
What I'm trying to get at is that this proposed investment may have some serious holes in the numbers because there isn't the predictability which could be used to ensure that the borrowed money gets underwritten by cast iron guarantees of work.