My experience has been that roofing estimates vary widely. I've had bids from $1500 to over $10,000 on the same roof!
Some companys charge more because they do more marketing than others, so they have to include the marketing expense in their cost when they calculate their bid.
There are some variables that a roofer would need to look at before bidding the job for your house. For example, if you have a 2000 square foot ranch style house, with a simple roof with only 1 ridgeline, you have a little over 2000 square feet of roof (the roofer will need to allow for overhangs).
On the other hand, if you have a 2000 square foot two story home, with a simple one ridgeline roof, then you have a little more than 1000 square feet of roof.
Then roof pitch comes into play. A roof with a steep pitch will take more materials than a lower pitched roof.
Anything added to the roof (dormers, hip roofs, etc) or if the roof changes directions, you will need more materials yet.
I would suggest speaking with no less than 3 roofers and check their references carefully. You can also check many businesses out at BBB.org, but don't rely soley on that information.
By the way, in my example above, the $1500 roofer did NOT get the job, neither did the ones over $10,000.