cost for heating and cooling a 2500 sf house.

I'm looking at a duplex that is split downstairs/upstairs. However, there is only one gas heater and one central ac unit feeding the whole house. I am just trying to get a ballpark price on running costs. I do NOT know much about hvac systems. The ac unit is big, but older and shows its age. The heater (based on a maintenance tag on it) is about 2 years old, again- don't know anything about it.

The house is in the Philadelphia pa suburbs and is about 2500sf. Built in 1901.

Feel free to chime in if you think it's worth adding a heating unit and then making tenants use window units/ or whatever you think is a better solution.

Thanks!