Read this article

http://www.wisedan.com/furnace. Based on their numbers, it would take 63+ years to make up the purchase of a new high-efficiency furnace. Plus, there is much more that can go wrong with them compared to a simpler mid-efficiency furnace, so you will likely see higher maintenance costs over the years.

Here's their calculation:

Your old unit gives you 80 cents worth of heat for each dollar's worth of gas it burns, and 20 cents is wasted. The new unit will give you 92 cents worth of heat for each dollars worth of gas it burns, with 8 cents wasted.

If your heating bill is currently $1000 a year, $800 worth of the heat is going into your house, and $200 worth is wasted by your old furnace.

With a new furnace, your house still needs the $800 worth of heat, plus the 8 percent that your new furnace will waste. $800 plus 8% equals $864. That's your new heat bill. You save a whopping $136 a year! That's more like 13% savings, far from 60%.

At that rate, your new system will pay for itself in only 63.8 years!
Plug in your own numbers to see if it's worth it .... likely not. Btw, I opted for a mid-efficiency new unit about 4 years ago. Purrs like a kitten