Work it out in "total" cost per operating airborne minute...On the ground the costs are generally much lower and fuel burn is often your only real concern (aside from what you charge back to the customer ie padding the margins). That would translate well to help determine your most efficient altitude. "Total" would have to include, Speed (TAS), Groundspeed (GS), Fuel burn, Cost per unit of fuel, Pilot/crew costs, MX cost per operating hour, A/C cost per hour (Payment or rental rate etc).
Using a website like:
www.fltplan.com you can calculate your optimal altitude in respect to time and a specific TAS.
You can build a fairly basic spreadsheet that can shift the TAS required to always allow you to flight plan for an optimum speed/cost based on a planned GS for your next segment, in other words taking head wind and tail wind components into account. You could also call all this a "Cost Index" that is used to adjust your performance of the aircraft.
Of course you would need to take into account the long term effects of operating at higher or lower power output and how it relates to the overall reliability(safety) and cost when it comes to overhaul.
DBW