What kind of servers are you running? What's your max QPS?
The fact is with your mmap impl. you probably use ram + virtual memory, and have more ram than needed to compensate for the fact that you don't keep the most used keys in memory, which a DB will do for you.
Point is if you have petabytes of data and access patterns only mean you access a subset of it, even Mongo might be cheaper to run.
So we are comparing here "just mmap" with "mmap + all that connection handling, query parsing, JSON formatting, buffering, indexing, whatever stuff that MongoDb does".
And no, MongoDB is effectively never a cheap solution. They are used because they are super convenient to work with, with all things being JSON documents. But all that conversion to and from JSON comes at a price. It'll eat up 1000s of CPU cycles just to read a single document. With raw mmap, you could read 1000s of documents instead.
In MongoDB conversion to and from raw JSON into BSON (Binary JSON) is done on the client (aka driver) so the server cycles are not consumed.
Mongo doesn't convert to and from JSON. The driver uses a binary protocol.