Datastore vs Memcache for high request rate game
As a commenter on another answer noted, there are now two memcache offerings: shared and dedicated.  Shared is the original service, and is still free.  Dedicated is in preview, and presently costs $.12/GB hour.
Dedicated memcache allows you to have a certain amount of space set aside.  However, it's important to understand that you can still experience partial or complete flushes at any time with dedicated memcache, due to things like machine reboots.  Because of this, it's not a suitable replacement for the datastore. 
However, it is true that you can greatly reduce your datastore usage with judicious use of memcache.  Using it as a write-through cache, for example, can greatly reduce your datastore reads (albeit not the writes).
Hope this helps.
I have been using the datastore with ndb for a multiplayer app. This appears to be using a lot of reads/writes and will undoubtedly go over quota and cost a substantial amount.
I was thinking of changing all the game data to be stored only in memcache. I understand that data stored here can be lost at any time, but as the data will only be needed for, at most, 10 minutes and as it's just a game, that wouldn't be too bad.
Am I right to move to solely use memcache, or is there a better method, and is memcache essentially 'free' short term data storage?