I can't give exact numbers, but we have a conversion rate in the (very) low single digits. It's plenty to support our costs and development team though (we're purposefully optimised for a healthy cash-flow positive business, rather than for massive growth). Our churn rate is pretty terrible compared to what most people would look for in a SAAS app, but that makes sense when you consider the academic yearly cycle, and that a lot of people only use LaTeX for a short period in their life while they are a student.

The collaboration features, only available on paid plans, are really powerful, especially when collaborating with "casual" LaTeX users. The Google Drive-esque experience is comfortable for even the most non-expert friends of mine. But I also wonder how many use these paid plans, they do though give large discounts to students.