Looking at Cloud SQL the pricing model seems off for such usage.
Assume an app such as Wordpress, Drupal, Joomla, etc deployed to Google App Engine for PHP.
Assuming it is a small website which will have sporadic traffic, it will cost: .36/day or 10$/month just to host the mostly dormant mysql database.
This is about the same cost as a low end virtual host, which will still be faster.
It seems to me there should be some option for hibernating an SQL instance and then re-enabling it - that way a mostly dormant site could leave the engine hibernating at the cost of a delay for the first visitor.