It is difficult to estimate how much an oil to gas conversion would cost. The location, type of equipment, and difficulty removing the old equipment factor into the cost. I would say you need to budget $15-20K for this project.

Assuming you don't need any ductwork (other than a supply and return adapter), you're looking at ~$6000+ for a "builder grade" (bottom of the line) setup. Google the online sellers and that will give you a rough idea of equipment cost, but I'm guessing equipment is around $3000 and installation will double that. Your location will determine cooling capacity needs vs heating capacity needs but I'm guessing around a 2 ton AC and a 40,000-60,000 BTU furnace.

Edit: This doesn't include removal of the old system.

Thanks for comeback that sounds great I can try ake out all old stuff I'll dig in more thanks again