Apple have a habit of charging fairly outrageous prices for extra RAM - $375[2] for 16GB in the current Mac Pro, and that's not even for 8GB sticks. With that in mind, $200 for the upgrade to 16GB is not that bad, which is probably on purpose.
[1] https://support.apple.com/kb/HT1270#link1 [2] http://store.apple.com/us/configure/MD770LL/A