Not sure if such a spread sheet exists but there is an excellent book by John Wensveen called 'wheels up'. It is worth the meagre investment and will give you a good grounding in airline business plan development.
You would have to take a look at the costs associated with the route.
-Gnd handling, landing, pax taxes at dept/arr
-Navigation fees
-Cost of fuel to operate the city pair(take into account fuel at dept/arr as the prices will be different most likely)
-What does 1 hr of flight time cost ? running the aircraft, crew flight pay, insurance, costs of maintenance.
There will be many sunk costs. There will be fixed and variable associated with running the operation.
These are just some of the main considerations, pprune probably not the place to be seeking advice am afraid.
Best of luck.