I would assume the actual cost of old 767's might be next to nothing....one-plane or two, everything but the cost of the airframe is nearly equal.
If they are owned, they have been paid-off and fully amortized at this point, if they are leased, the leasor might be willing to let US fly them for cost of maintenance.
If they question is, could 2 airframes serve 2 slightly closer destinations that allow for a better return, then I would assume that analysis has been done, or that US is willing to make a marginally smaller return to be in high-profile markets.
But since you asked it from a pure "money making" point of view, I'll contend that very little of the CASM is tied to the actual second airframe.
If they are owned, they have been paid-off and fully amortized at this point, if they are leased, the leasor might be willing to let US fly them for cost of maintenance.
If they question is, could 2 airframes serve 2 slightly closer destinations that allow for a better return, then I would assume that analysis has been done, or that US is willing to make a marginally smaller return to be in high-profile markets.
But since you asked it from a pure "money making" point of view, I'll contend that very little of the CASM is tied to the actual second airframe.