It completely depends as to what kind, oil or gas as well as fhw or fha and then add on if you need baseboard or ductwork. The best bet is to get a couple of contractors in to get a free estimate. That way you are comparing appes for apples.
Of course a professional HVAC company could be exact with their numbers, but I would say you should expect to spend between 5-7K for a new system in a home of this size. Good luck! By the way, a new heating system is a very wise long term investment in your home.
Heat system as in duct work and new burner or just replacing the burner unit? Gas or oil? There are so many variables here it's really hard to say. Your best choice would be to contact a local heating/plumbing company and give them the specifics.