Here is a place that will calculate the savings for you.
Go to www.fueleconomy.gov
Once there, at the top left you will see an option for "find a car"
Pull up the 2011 Ford Fusion 4cyl 2.5L
Once open then choose the option to "compare"
Now pull up the 2011 Fusion Hybrid
In the mid section of the comparison you will see an annualized fuel cost.
$2088 for the 4 cyl
$1338 for the hybrid
$ 758 savings for the year or $63.16 savings per month.
I suspect that the price difference to purchase the hybrid is more than $63.16 per month??
These annualized costs are based off the assumption of 45% hyw and 55% city at today's prices of $3.48 per gallon at 15,000 miles of driving per year.
NOW HERE IS THE BEST PART:
In the fine print under the dollar amounts there is a little hidden link that allows you customize. That way you can see what it would be if you were 80% hyw (which btw reduces it to only a $453 savings or $37.75 per month)
At this 80% hyw settting, compare in a 2011 Hyundai Elantra 4 cyl and the Fusion Hybrid and the Elantra comes out at an even tie!
If you want want to know what the savings would be if gas hits $4, it can do that too. It also adjusts for the increased price of premium or diesel
I do like the Fusion and really like that you can get nice things like leather and climate control w/o having to buy the V6 engine.