This is the only page I've found on pricing so far (https://docs.mms.mongodb.com/core/api/)
As with changes made through the web interface, changes made through the API are subject to MMS pricing. If you add servers beyond the provided number of free servers, you must have a valid credit card on file with MMS or risk having your account locked.
The 2 links don't work - they take me to the generic management page
both redirect to https://docs.mms.mongodb.com//tutorial/nav/monitoring/