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It would take about an hour......hour and a half... to install, so if you can get it done for that price, I would go for it.
Obviously I cannot speak for the "legit" side of things, so you will need to take that up with the person that will install it.
The $40 will be a cash price, and if you were to have any issues with the install like wind noises or water leaks you most likely won't be able to find him again, so you will take a chance having it done this way as opposed to a reputable shop that will guarantee their work.