Hi! I'm one of the authors. Good point, we should add something on that. Briefly our mongo db setup was running on 3 r4.xlarge instances which comes to around $5000/year with storage, plus the cost of opsmanager, which I think was around £40,000 including the support contract.

In Amazon it's harder to compare directly as the support contract is paid across all of our accounts, but we're spending around $13,000 for a highly available db.r4.xlarge postgres instance.

Performance wise, querying without an index is SLOW, basically not worth it - as we end up doing a scan of the entire database. Fortunately we don't need to do this as we can usually rely on the guardian content API for proper searching stuff. The average API (a Scalatra app) response time reaches 150ms at 'peak time'. This isn't a high-performance use case - around 1000 requests/minute at 'peak' time.