Welp, nobody can blame you for wanting to get away from Nagios. It’s certainly a tool from a different, simpler era and hasn’t aged well in our opinion.
As a push-based metrics solution, Librato is probably a lot different than what you're used to. But don’t worry: we're super easy to get up and running with, and obviously you no longer need to worry about maintaining or scaling infrastructure. Also, unlike with some other solutions, you can use us with your existing toolchain (it’s easy to plug us into your existing Nagios infrastructure to try us - the trial is free & full-featured).
We’re a hosted metrics platform, meaning you can send metrics of any type and amount you want. We’re functionally similar to Graphite+Grafana, except we do all the work of scaling and management for you so you can focus on the metrics themselves. We provide alerting and other useful bits out of the box (things that are not trivial to setup yourself, e.g., bolting together collectd+Graphite+Grafana+statsd+flapjack+kitchen sink and hoping it scales and doesn’t fall over). We’ve got an agent that comes with a bunch of turn-key integrations too, to make it super easy for you to monitor what you care about.
As to pricing, we're the only hosted monitoring system that will just charge you for what you actually USE. You pay pennies per metric metered by the hour, instead of a per-node model, which gets crazy expensive and inefficient for modern ephemeral infrastructure. For example, if all you're doing is integrating us with AWS CloudWatch to monitor some EC2 instances and an RDS instance, we can do that for effectively a $1-$2 an instance. We also have an agent you can install on your servers if you want more detailed metrics, which adds $5-10 per instance depending on how many metrics you enable. Our customer success team (email support@librato.com, or the Help chat window if you already have a Librato account) will be more than happy to walk you through any permutation of our pricing and the details of the model to help you better understand it.
As mentioned, you can try us out for free--no credit card required: https://www.librato.com/