i believe there are 2 threshholds you need to surpass in order to increase the bonus.
if you pass the first, you get half the bonus (see http://puu.sh/wlufO/b1da593a65.png )
then there is another one, if you pass that, you get the full bonus. no steps in between. I do not know, how much you have to wager / how many tickets you have to buy. Maybe someone could test this?