Personally, I believe we're *all* smart priced - the smart pricing is built into the entire system. And sometimes that works to our advantage, and sometimes it doesn't.
Advertisers don't want to pay full price for clicks on sites that don't convert. That's why Google came up with smart pricing.
Whether or not you panic is up to you. Me, I'd be looking to see if there are any patterns I can spot as far as what has dropped - is it sitewide, or over specific pages, is it a particular type of traffic, or a particular ad unit.
There's not a lot you can do, but you won't know until you figure out what might have changed.