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?
It's less about the hardware and more about the DBA service. Most companies shouldn't be spending engineering time setting up DBs and don't have the expertise to run them when they're "catastrophically successful". We do, know what the watch for, and have on call DBAs that respond to issues within about 10 minutes, 24x7.
Ultimately, MongoHQ are pros and you're paying for their time and expertise, I am pretty sure if you were going to buy consulting time from them it would cost a lot more than what they charge for their service.
And the plans also start with so little space
The number of times I've seen a database go down in production either because it's solitary mirror failed with no one noticing or because it wasn't consider "mission critical" enough to have a mirror... you're not just paying for the hardware, you're paying for the guys managing the hardware. If you're a company with databases and you don't have someone on staff whose primary job is making sure your databases still exist from day to day, one day they won't.
For us, saying, "oh shit, a handful of other people had a bad experience – we should drop everything and move our data to postgres" would have been a bad decision and a waste of time/money.
By default Mongo uses MD5 for password hashing (with username as seed) so it's not secure to use it over a public network.
(if your server is co-located on the same network as MongoHQ then you can probably hack it using network access rules)
SSDs also help with writes and can help mitigate the infamous global write lock problems in Mongo.
Heroku's postgres is just as (or more) expensive as mongohq/mongolab, but they don't limit storage at all (except at the dev tier plans).
Database storage space correlates pretty well to the management complexity, at least with our customer base.
Offering unlimited storage on a DB is somewhat disingenuous, there are effective resource limits that matter long before disk space and those (RAM/IO capacity) are the things that cost actual money.