Best key value database if I’m optimizing for low cost?

I have an application where I need a dirt simple key value store. The app is very read heavy (10-1 or 100-1) with absolutely no need for any queries or filtering. It’s only key value look up.

Data durability isn’t a huge concern here and eventual consistency is more than ok.

This is for a startup project I’m bootstrapping with my own money, so I’m optimizing first and foremost with cost.

SQL is unnecessary here as I am okay with data loss and the data isn’t at all relational.

Cassandra, redis, and mongo are my fist hunches.

Cassandra: Cassandra performs better for writes than reads. Cassandra is technically key key value which is unnecessary for starting off. Could be useful down the road for sharding but don’t need it now.

Redis: seems the most appropriate. Biggest concern is it’s in memory and RAM heavy instances are more expensive than HDDs. Given I care more about cost than speed… I dunno if this works.

mongo: not familiar with. simple json/document storage would work, but don't know if redis/cassandra would be better.

Thoughts? thanks!