Well, if we make it like a 0.000,001% chance I will give it a +1. As it would statisticly be like a single person a year assuming the server get 3000 votes a day. Even if we double or triple it I will give it a +1 as it won't take much away from donations, and more people would be encouraged to vote every time someone got a free rankup.
But honestly the current sytem is fine enought, but it isn't well enough advertised (that voter of the month get a rankup, if they get it at all, I just see other people saying that). Another good alternative is to make a bit bigger deal out of the voter rankup thingy, and advertising about that the voter of the month get a rankup.