You're right when you say that you get what you pay for, but...
If you use varnish A which costs twice as much as varnish B, you will not get twice the use out of it. But remember that a lot of this is the work it takes to apply the varnish, which is why I decidied to try poly, the really nice kind. They use it on Rive boats (they look like the old chris crafts)and they have 15,000 dollar paint jobs, with only 1,000 dollars of paint.