I put my own furnace in a few years ago, replacing the original. The first unit I bought was so much smaller in size than the old one, that I could not figure an effective way to hook up the old ductwork. Fortunately they sold the same furnace in a bigger "package", and allowed me to return the first one. It's worked fine since day one, and my gas bills were 1/2 the first year. Of course, gas prices have almost doubled since then, so I'm pretty much back where I started monthly cost wise, but I have a new furnace!
Oh, cost me about $700 for a new lennox, 3 years ago.