Also a great plug for Jekyll in general. While there can certainly be more worrisome factors than cost for a site, site performance should be one of the most important issues when launching. By completely getting rid of a backing database, page lookup times can be reduced dramatically. As Prose continues to mature, it's getting more and more flexible by the day. Just last week they implemented an extensible "tagging" feature for posts: https://github.com/prose/prose/issues/464.
In case you haven't noticed, I'm a fan. Kudos to dev seed and big props to the White House team for the willingness to move away from Drupal.