It might be really expensive because there may be no good spots to put the ducts in. I would still hope that you could have it done for $20k or less. Expect to have to tweak the vents to balance the heat or cooling to your liking. Of course without seeing the layout, I can't be sure. I had my last house switched over for about $8,500, but it was a 1300 sq ft single story, and pretty open. I also had a limited amount of vents, but in this design it did work. I will say that I was much happier with the forced air than the boiler, and I did have to put a new boiler in about 5 years earlier...You will be smart to get a few quotes before writing an offer, or write in your offer that it is contingent on getting an acceptable quote.