We don't set prices for add-on providers offerings, but note that Redis is an in-memory database and that memory is more expensive -- per GB -- than disk. (I'm an AppHarbor co-founder)
But Redis should have at least as much RAM as you have data and will only use the data from RAM. You cannot swap or store it on a slower disk, while Mongo can use a disk file.
Redis can backup or journal to disk, but this is just so the data is persistent between restarts, not used in queries or runtime.
* Updated based on comments: I originally wrote that redis requires as much ram as data (technically more) but I'm correcting this to state that Redis CAN use disk, but Salvatore, the man behind redis, publicly states that he does not recommend it as a solution.
This is incorrect. Redis can swap out to virtual memory when it runs out of RAM. Although, it should be leveraged with some care (http://news.ycombinator.com/item?id=2606096)
Thank you for the link. I actually do remember reading that, and I guess I completely forgot it was even possible. After reading about it, I told myself that redis will be in-memory only for the solution I was using it for. I'd definitely be curious to see some real world performance numbers for redis using the disk VM before considering it an option.