Depends on what you want to monitor. Grafana is pretty decent, but the real draw to Datadog is their APM stack. The UI for tracing and looking at stuff is pretty awesome.

Though you could get most things into Grafana with something like Prometheus. The problem with Grafana is understanding what the limitations are. If you're not careful with the number of panels and such it can get quite slow.

I've used Grafonnet before for doing Grafana at scale. Simply put, I hate it. Apparently an alternative is being worked on at Grafana so I'm waiting for that. But if you need to make hundreds of panels....it works well enough.

If you need to monitor some infrastructure you can just use Telegraf and output it to Grafana if needed. It kinda falls apart though because another great benefit of something like Datadog is not managing a time series db. That can get ugly real quick.

I guess it all just depends. If my bill was super high I wouldn't mind spending some resources on Prom/Grafana if you're in the Kube space or some Telegraf/InfluxDB if you're not.

I've also heard good things about Timescale but haven't used it.