There aren't many instances of actual fines, but this is the most prominent one: The FAA tried to fine drone pilot Raphael Pirker $10,000 [1] for being paid by the University of Virginia Medical Center to film a promo video with a drone. Pirker challenged the fine and won. The FAA changed its rules after that decision and Pirker eventually settled for $1,100.
[1] http://kernelmag.dailydot.com/issue-sections/features-issue-...