1TB costs $6000/month at your pricing. 10s of TB is rather expensive. It's great if you can get those rates but I'm having trouble seeing how these numbers can work on large datasets.
You are right in that it uses SSD and indexes under the hood to power the SQL queries. There are volume discounts available at higher storage volumes in the 10s of TB range, so, the cost wouldn't scale linearly.
Rockset builds multiple indexes to enable low latency SQL queries that can be served directly into applications. This might not make sense for say - storing a lot of log data and querying it rarely, and makes more sense for data that needs to be actively queried with low latency.
