The longer a surfer stays a member, the more the monthly memberships costs them?

I know a lot of sites offer discounted rates the longer a surfer stays a paying member but i was wondering if anyone had tried charging more the longer a surfer stays a paying member?

I mean, in theory, the member should be getting charged more if you think about it, they get more and more content each month on a site if they have been a member for more than 1 month than a surfer that joined today for example, shouldnt they have to pay more because of it?

What are your folks thoughts on this, have you ever tried upping the membership price after the members initial month and, if so, how well did it work?

Regards,

Lee