Often money is not the best motivator. Dan Ariely's research shows people who were asked to volunteer consistently put in more effort than those who were compensated with money or gifts. There's a whole chapter about social norms vs. market norms in his book: Predictably Irrational. There's a decent summary of the chapter here: [1].
[1] http://robertnielsen21.wordpress.com/2013/01/15/predictably-...