Hi Gero
MongoDB is scalable so it works a bit differently for that than for MySQL or PostgreSQL, if you click on the "Full Details" on the price point you were looking at, it does say "10MB RAM & 6 concurrent connections for Redis, MongoDB, and RabbigMQ instances" - so each instance of MongoDB can be as large as 10MB. Does that make sense?
~Tracy | Appfog | Tech Support Engineer