As stated by others, there are question to ask. When I moved into my house, we had oil. I had it changed to gas. Costs were (approximately as it was a few years ago):
1) Have the gas company run the gas line down the street for me - $5,000. If the gas line were already at the street, it would have been free to run the line to my house.
2) New furnace/AC unit, outside compressor, electrostatic air filters, humidifier, and water heater $8,000 (or so) - all installed.
It's good to be back.