I've kept some data on how much time I've spent on a somewhat popular open source project (+/-1000 stars on github, if that means anything).
Time to implement to scratch my own itch: +/- 24 hours
Time to write documentation, package, etc: +/- 70 hours
Time to handle bug and feature requests, support: +560 hours
So that's an overhead of around 2500%. If I was paid to do that, it would have cost around €130,000.- (and would have netted me around €5, because dutch taxes ;-) )
The emails and other forms of thank-you's from users make it more than worth it. Plus, I get to give back to the community.