I built an online registration system as a favour to organisers of 2010 Australian Masters Championships in Perth. I did it for a few reasons:
The system was well liked by the organisers and also got some nice comments from end users. Behind the scenes the system was put together quickly with no flexibility. There was no ability or expectation of other users.
I earned no money from the use of the system. All payments went to the Paypal account of the national club. Later the organisers asked for a version they could use annually for their state event. For this I did receive a flat license fee.
However I had young kids, was looking for financial stability, had not come up with a business model and was burnt by a previous attempt to build a business. So the system remained largely untouched form my side.