Seems like if this is voted for positively.. than you will almost certainly HAVE TO pay the 6 dollars to increase your chances. depending on the number of people who do that, it will either negate the effect, or push all of the "common" plots to the back of the line and to the people who dont pay the $6.
So depending on how many people do it.. seems like if you do it without, than you are almost guaranteed to get a common.