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The CPMs on event related things might be high enough to just rely on ads.

http://mixergy.com/evite-harry-lin/ "effective CPM, when I left the company, we got, we’d driven it up, the effective CPM was just under ten dollars. And on certain pages, certain classes of pages, we enjoyed a one hundred dollar CPM, on all those ads"

I can imagine that, for example, TGI Friday's would pay a decent amount for an ad on a meeting request to eat out at Chili's or Ruby Tuesday's.

Keep in mind that it's really hard to approach these types of companies. And when you do approach them, you'll have to have a large audience.

Depending on the price range I would. My fiancee and I already use a shared calendar but it is woefully equipped for the heavy use she and I put it through.

All I can tell you is that it's something I'd definitely use, and something many of my friends who I've told this to over the years would use. That's not saying much, sure, but if you could provide a valuable and useful service, the demand will certainly follow.