pricing an addition

I have a 18 year old 2 story frame home with vinyl siding. It has 3 bedrooms, 1.5 baths, LR, DR, and deck. The home dimensions are 24x28 giving me 1344 sq feet. I am considering building a 14x 28 addition onto the back of my house where my deck currently sits. I would like to know the different ways of estimating the cost of the addition. Are there any "home improvement calculators" on the net or any websites that have the resources to estimate the cost per square foot?

Thank you,

b. cross