FooBarWidget (who you replied to) is actually Phusion, the maker of Passenger (the very popular Rails app server). I think they have some very strong anecdotal evidence.

Up until recently Passenger was completely free, with donation options. Now they have an Enterprise offering - which we are happily paying for. I suspect the Enterprise licenses are orders of magnitude more than the donations ever were.

Human observation. How many open source applications do you use that you have paid for?