Appreciate the feedback (librato co-founder here), we should definitely make that clearer on the pricing page and elsewhere. We're in the process of adding some content around that now.

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".