No direct experience with your proposed arrangement but it would seem very complex in that you'd be renting floor space, power, utilities, dust collection, and then separately all the other shop equipment usage. Basically offsetting your rent with any production you supply to them. I can only imagine unless they wind up burying you with work you will likely owe a decent sum every month hopefully offset by other work you source. They are basically going to get you and the machine for pretty short hourly money.
First light, it would seem they just may be the type that is not willing to implement CNC on their own which may be a good thing in the long run for you in hopes once they see the benefit they dont just oust you, buy a machine, and go it alone. I suppose it could be a win win they have zero risk/investment other than the loss of floor space if they just have no interest whatsoever.
If it were just sheet processing it'd be much easier for you to calculate a projected cost per sheet/per job and that would depend a lot on how standardized their product line is. Highly complex, long run time, lots of machining, machine dependant, sheets are not going to cost the same as a basic panel processing. When sheets go from 4-6 minutes each, up to 10-15 or more, its not the same game.
Beyond that factoring a per hour overall shop rental rate (provided personally and legally they are truly comfortable with handing you a set of keys and turning you loose) wouldnt seem hard.