While I don't know it's x4 worth it, I can personally testify to how expensive anything FIPS 140 is to get done..
"Because the AWS GovCloud (US) Region is physically and logically accessible by US persons only and also supports FIPS 140-2 compliant end points, customers can manage more heavily regulated data in AWS while remaining compliant with federal requirements."
Keep in mind that this means they're operating one or more facilities exclusively for GovCloud. Since these facilities are almost certainly smaller than the rest (and, as you mentioned, they're more work than normal), it only stands to reason that it'd be more expensive to use.
"Because the AWS GovCloud (US) Region is physically and logically accessible by US persons only and also supports FIPS 140-2 compliant end points, customers can manage more heavily regulated data in AWS while remaining compliant with federal requirements."
http://aws.amazon.com/govcloud-us/