Roof Design Considerations
  Roof Pitch is the number 1 factor in roof design

A roof with a 12/12 pitch that looks aesthetically  pleasing can add as much as 50% or more to the initial cost of a roof when compared to the same house with a 4/12 pitch.

Why does a steeper pitch add so much to the price?

Here we are going to compare a 12/12 pitch to a 4/12 pitch.
12/12 has 37.5 % more roof area 
12/12 roofing labor is 50 % + per roofing square
Rafter and sheathing labor is 50% more
Roof framing lumber is more

Other factors that add to the increased price can include:
Taller chimney
More attic ventilation
Increased painting costs
Increased stucco or siding costs
Roof maintenance costs, although the need is less frequent. 
On the plus side most roofing materials can last 50% longer

 This means that if you are going to use a steeper pitch it would be wise to use one of the better top end roofing materials available.