This is already a product that exists many times over. Besides the aforementioned wget, I've recommended less technical users to SiteSucker, a Mac/iOS app.
I'd be happy to bill your clients to do it for them though!
I'd be happy to bill your clients to do it for them though!
They want to write a check and get back to business, not become a web developer!
(I think that tool is awesome though, and I appreciate the tip!)