Having worked with some quite large Mongo setups, I think you'll find the administration of Mongo ends up being significantly more than a single upfront cost.
Dealing with sharding + replica sets is an administrative nightmare with Mongo.
Do you have some more insight into this?
We currently run a hybrid Riak-MySQL setup, and are actively looking at Mongo to replace parts of it. In a few tests mongo 's speed seems to be all over the place: From REALLY fast to quite slow. And that is without the admin involved in a larger scale setup.
Dealing with sharding + replica sets is an administrative nightmare with Mongo.