For example, you can get a 2 core, 2GB memory, 20GB of SSD backed storage from DigitalOcean for $20 a month. So, buying three VM's from Digital Ocean for a replica set is a total of $60 a month. Essentially the same thing at MongoHQ is $500 a month.
I completely understand that you pay more for using a PaaS, but that much more?