We were using New Relic APM and Server Monitoring through the Microsoft BizSpark benefits for an year or two. We were even ready to pay for their licenses, but New Relic pricing for multi-server use was starkly high for a small tech company like ours. Their product was good, but their focus was more on enterprise customers.
After trying out many other commercial products (focusing on .NET Core stack in particular), and keeping long-term scalability and TCO in mind, we decided to utilize some open source solutions.
First we tried Bosun[1] (from the Stack Exchange team), but found little progress being made on it. Then, we tried Prometheus[2] collectors for monitoring, Grafana[3] for the dashboards and alerting, and Exceptional[4] for logging. One person got it all working within a week. We've never looked back, and have to hardly touch the set-up anymore (except for system updates once in a while). It has worked beautifully for almost two years now, going from 2 to a few dozen apps/servers/VMs.
Hope it helps you, and others, explore the open source monitoring route.
[1] https://bosun.org [2] https://prometheus.io [3] https://grafana.com [4] https://github.com/NickCraver/StackExchange.Exceptional