When a sponsor takes over a whole page, his message is less rushed. When his message is less rushed, it's often better read, which means people give it more time and attention. If you can build the content to attract the kind of people your sponsor needs, you'll be able to show cost per lead to be lower and quality per lead to be higher. Advertisers will have no trouble paying for that. As opposed to random hit sites, a sponsor who knows you're targeting just for him will actually help you with content development.
Sponsorships are multi-dimensional deals.
You can create added value programs for your sponsors, too. Stuff that involves mentions of your site for an extra value, which proves the effectiveness of your site. The one thing that really puts off advertisers is not knowing where their ads are running at any given moment. For just this reason, whenever we schedule a client to run outdoor boards, we always make it a point to post one billboard right outside the client' s office, just so he can see it out his window. Advertisers like to know they can click on a site and see their stuff.