When I looked at MixPanel we were already at more than 8 million page views per month and wanting to track more events than that.
But as we wanted to try MixPanel, the best way was to start by sampling a small number of users and activities.
As our trial of the software progressed we found we got way more value out of sampling a smaller set that we could make sense of rather than measuring everything.
For us, MixPanel became one more tool in a wider toolset. We didn't see the value in jumping up to the paid level as we didn't feel it would replace all of the other tools.