Is this vinyl siding or fiber cement? What kind of defects do you have?
Usually vinyl siding is "defect-less" and problems come for band installation, such as siding being nails too tight to the wall. This is easy to detect - you will see warping / bubbling in the siding.
As for cost to replace, this may help you - http://www.sidingcalculator.com
Repair costs are different and depend on extent of damage, size of the job, materials, difficulty of access, and other factors.