I don't see any reason why I make it for free or even open source. I charge my customers monthly fee is not just based on how much time I spend to build, it's about how much commitment I have to keep it working to provide my customers high quality service.
If you think give tips to a waiter/waitress after they serve you meals and drinks makes sense, and then, why I provide my service for free? My pricing mode maybe need to judge a little in the future, but free is never my option.