I think it's important to define exactly what you want from the designer. If you're looking for a graphic artist to come in and provide a slick look and overall template for the site, my guess is you could get by with just a few templates to build out from. 20 to 40 hours per page for that kind of work strikes me as excessive.

However, if you're looking for a user interface designer who will put some real consideration into what goes inside the template shell ("epicenter design" as coined by 37signals), then assume more hours. Ideally this person would want to be there to adjust or tweak elements or flows as you analyze your traffic and success rates.

I have a client who usually knows exactly what he wants from a content/layout/interface standpoint, so my work becomes more graphic production. This generally takes under 3 hours per page. But, we've built up a visual vocabulary for the site over a number of months.

Other projects that are starting from scratch may take your aforementioned 20-40 hours up front to arrive at an agreeable overall design direction. But it usually shouldn't take that much time per page.