[otisg]

Any modern monitoring service will provide you with volume discounts if you have hundreds or thousands of servers.

Since you mention Papertrail specifically in the context of costs - Papertrail is actually a bit pricey relative to the competition. For example, compare https://papertrailapp.com/plans to https://sematext.com/logsene/#plans-and-pricing . I think Sematext Logsene is 2-3 times cheaper than Papertrail.

Lastly, I was at a Cloud Native Conference in Berlin last week. A lot of people have the same setup as you - ELK for logs + Prometheus for metrics. We're running Sematext Cloud where we ship both our metrics and our logs, so we can easily switch between metrics and logs much more easily, correlate, and troubleshoot faster. Seems a bit simpler than ELK+Prometheus...