They "charge $0.000002 per measurement. As simple as that." -> https://metrics.librato.com/pricing
In short a measurement is a single datapoint i.e. <key, value, timestamp>. So if you have a sense of how many metrics you want to track and at what frequency, it's relatively straight-forward to calculate the list price. There's an estimator on the pricing page to help you do that once you understand what constitutes a "measurement".
That said, librato seems like it would be nice if your stat volume was low.