Does anyone know how much it would cost to create an extension that allows people to save products that they find on my site, so that they can open up their favorites list at any time from the browser?
Making an extension on Chrome is very easy. As long as you know basic javascript and Chrome objects, you can make decent Chrome extensions yourself. I don't know about Firefox though, but it is very easy with Chrome.
http://lifehacker(.)com/264490/how-to-build-a-firefox-extension I found that tutorial extremely helpful when I looked into it. It seems Firefox is much harder than Chrome.