Looking at Google's Terms of Services, it does not look like you have to pay Google for the use of MapKit with Commercial applications.
Google Maps Terms of Service (Specifically for the iPhone)
http://code.google.com/apis/maps/iphone/terms.html
However, there are important restrictions such as you cannot charge a user for solely the use of a map (which most likely means if your app is just a locator application, you cannot charge for it).
