There are some limits on what a Worldcon can charge. They cannot charge more than 125% of the Advance Supporting Membership (voting) fee charged for their election until less than 90 days before their convention. That is, until 90 days out, they can raise the price to not more than 125% of their voting fee (Denvention raised their supporting membership price on January 1), and at 90 days out, there is no limit and they can charge whatever they want.