Replacing roof shingles has at least two costs,
the cost of removing and disposing and cleaning up the old shingles.
And then the cost of replacing the roof with new shingles.
In both cases you will have to pay for labor,
and the longer it takes the roofers to remove the old shingles,
the more expensive the total job will be.
Perhaps you should agree to a firm price before they start the job because often there are surprises that the contractor might want to spring on you.
If you previously agreed to a price and have a contract,
then it doesn`t matter how long they take.
Expect the labor to get rid of the original shingles to cost you 1 to 2 dollars per square foot.
The actual installation of the new shingle should cost you about 5 per square ft.