There is no one answer to the question of how much it costs to replace a roof.

When seeking estimates from different roofers, make sure that you compare apples to apples.  Each roofing company offers their own individual craftsmanship, experience, and quality of materials.  If one contractor is lower than the next, the best assumption is the experience of their roofers is lower and they are paid at a lower rate.  Also take into account the quality of materials, the items being replaced and the length of warranty you are receiving from each contractor.

If you are on a budget, inform the roofer so he can include low cost recommendations in the estimate.  Professional roofing contractors will provide you with three price options with low, mid and high grade materials so you can choose your own price. 

Roofing estimates are based on multiple factors, including square footage, pitch, height, layers, material quality chosen and time-frame.

The roofer may also charge more depending upon the slope or angle of your roof, as this may need additional materials to cover.

The roofing contractor may need to remove the prior roof which will add to your total cost of your roof replacement, including the cost to dispose or recycle your old shingles.

Do not attempt to remove the roof yourself in an attempt to save some money. This type of work should only be done by an experienced roofer.

If you are wishing to save money by installing your new roof over your existing it is a possible option.  Ask your roofer to determine how many layers and what your local code limits your house to having.  The very best option is always to start with a fresh roof sheathing area, but that doesn't necessarily mean you have to spend the extra money.