Lots of people use Redis at a cache, not a primary data store. You can have full querying in your SQL database and fast access to common requests using Redis.
In what way is it too expensive for side projects? It's the easiest data store to compile and run that I've used.
How are 3rd party hosting options part of a cost comparison?
Hosting any DB offsite comes at a cost, and it does not appear that any one database platform has an advantage over another in terms of a service provider.
My current little side project uses about 200-300 MB of database storage. Using Heroku, that would cost (pr month) $9 on their shared postgres database, $10-15 using mongodb, $50 using heroku's dedicated production postgres service, or $125 using redis.
I stand corrected. I was using the prices from redis-to-go, which was the only available option last time I checked, and the service with the big prominent link at the top of the heroku pricing page. Good to see that there is some competition in the area and that prices have come down a bit. Still not cheap enough to move my hobby project from a 1GB VPS, but reasonable enough if I ever turn my project into something that might make money.
First, running redis remote where you have WAN times involved is typically not the most performant. Second, if it's the cost, run it on a VPS where you'll get more memory for the money (hopefully the same VPS provider you use for other things).
In what way is it too expensive for side projects? It's the easiest data store to compile and run that I've used.