All our development and staging systems are essentially free, mongolab/Mongohq have free plans that are enough for dev. That being said I'm definitely not looking to save 20$ as I bill 110€/hour so one saved hour I can devote to actuall work pays for half a year of payed heroku dyno;)
I'm looking at migrating the whole thing to Heroku but the Mongo cloud providers seem to cost 2-5x more.