How do new furnaces differ from old ones ?

How do you calculate the pay back period for a high efficiency furnace using graphs?

Do the math, with an 80% unit the operating cost is around 20 to 30 % more expensive than a 93 to 95% unit
Guessing, a $100.00 gas bill would drop to $70 or 80 $$,, probably even more
Say you save 120 bucks a year,, it might take you 3 .5 years to save the extra bucks you spent on the high efficiency unit

This my friend is an uneducated guess,,

The bad part is the fact that when the new high efficiency furnace has a problem you will spend more than your savings on your gas bill to fix it.