When you're shopping around for a car, you're looking for the best price on a the car you want. You already know what brand and, likely, what model you're interested in. The dealership is just a necessary evil between you and your new vehicle. The less of your cash you end up having to fork over to the middleman, the better.
But when you're shopping around for a dentist, you have different priorities in mind. There isn't a mass-manufactured product with a corporate identity standing behind it. Instead, every bit of dental work is a custom job, a craft being performed by a single person with a drill and glue. It may cost less to have the same procedure performed in a 3rd world country, but any qualified dentist can show you just how different that "same procedure" really is.
A website, like dental work, is a customized service performed exactly to your needs. It is not a one-size-fits-all product that you can "find a good deal on." As for having your site constructed by a friend, relative or other non-professional, again, think dental work. If getting a solid website up is important to you, take it seriously, hire a professional.