What I can say from my own experience: last season I started with no win bonus, my team did well but then lost and drew a few games in a row so I put a medium win bonus and went on a 18 games winning streak till the end of the season! Oh, and I never have win bonus at max. My strategy is for win bonus+player wages to be less than league match prize. But that might be a very conservative strategy and I don't know if it works on higher levels.