The answer to 'cost to replace underlayment in my area' is that the cost can vary widely depending on several factors. Generally, the cost to replace underlayment can range from a few hundred dollars to over a thousand, depending on the size of your roof, the complexity of the project, and your local labor and material costs.
Some key factors that influence the cost include the type of underlayment material (e.g. felt paper, synthetic, or self-adhered), the accessibility of the roof, the condition of the existing underlayment, and whether any additional roof repairs are needed. It's important to get quotes from local roofing contractors to get an accurate estimate for your specific situation.