Read Steve McConnell's Software Estimation book, and learn how to estimate well. Then estimate the project with an uncertainty range, and either provide the unc

Read Steve McConnell's Software Estimation book, and learn how to estimate well. Then estimate the project with an uncertainty range, and either provide the uncertainty range in your quote (if they can handle that) or just give the 90% or 95% confidence estimate, depending on how much risk you're willing to absorb if you run long.