Currently, when you cast a site selection ballot for Worldcon, you must pay what is technically known as an Advance Supporting Membership Fee (but what almost everyone calls the "Voting Fee"). That makes you a member of the Worldcon two years hence and allows you to vote on where it will be held. If you vote, you automatically get a supporting membership in that Worldcon. (That's why it's called an Advance Supporting Membership.) You also get the right to convert that supporting membership into an attending membership for not more than twice the amount you paid to vote for at least 90 days after the election. (In other words, voting purchases an option on an attending membership.) I'm going to use "ASM" to mean "Advance Supporting Membership (Voting) Fee" hereafter.
Worldcons don't want to sell memberships that cost them more to provide than they receive in revenue. That's understandable; no retailer wants to sell a product or service at less than the cost of goods sold/service provided. With some knowledge of what it costs to put in a Worldcon, most recent Worldcons have tended to charge the maximum amount they can to convert to an attending membership. Moreover, since the ASM sets an effective cap on attending membership cost for at least 90 days, bidding committees tend to want to push the cost of the ASM, and consequently the Worldcon supporting membership, higher and higher.
An analysis of the cost of providing the services in a supporting membership leads to the conclusion that supporting memberships are vastly overpriced. While part of the purpose of a supporting membership is to support the Worldcon, you could probably sell a supporting membership for $25-$30 and still produce revenue in excess of cost-of-provision-of-service to help support that Worldcon. And the current supporting membership cost is clearly driving people away from getting involved in any sort of participation in Worldcon, such as voting for the Hugo Awards.
With your initial attending membership prices set as a function of the ASM, even if a committee wanted to make it cheaper to join as a supporting member, they couldn't do so without cutting their economic throats on the cost of an attending membership. The price cap used to be even lower, and it was a pretty good rule of thumb that Worldcons were losing money on every early attending membership they were selling. Essentially the regular participants in WSFS, by setting up the price-cap rules, were giving themselves a below-cost membership subsidized by anyone who bought his/her membership later.
Some people have suggested raising the multiplier (currently 2x) to a higher value, such as 2.5, 3, or 4x, so that committees could agree to lower the ASM while still collecting the same attending membership. Others have said that if you raise the multiplier, committees would immediately jump the initial attending membership to the maximum allowed value. That's about what happened when the cap was raised from 1x to 2x the ASM, after all. However, I think that's because 1x was entirely unrealistic and committees hadn't yet twigged to substantially raising the ASM anyway.
My idea is a bit more radical: I propose that we eliminate the price cap tying the ASM fee to the initial cost of a Worldcon attending membership. Let Worldcon committees charge anything they want for their attending memberships, regardless of whether the person voted or not. (You'd still want to specify that supporting members of any stripe get credit for whatever they'd already paid when they upgraded to attending, I guess.) Then start pressuring bidding committees to lower the ASM and pledge lower supporting membership costs -- say in the $20-$40 range. Criticize committees that won't commit to such a cost. We currently apply such moral suasion to get bids to agree to participate in Pass-Along Funds; why couldn't we do the same to pester them into offering more-affordable supporting memberships?
I suggest that the target cost of a supporting membership should be approximately the cost of a new SF/F hardcover book -- not much more, and probably not much less, either. I think this would prevent causal ballot-box stuffing (the usual bugaboo raised when anyone wants to lower membership cost) while not scaring off so many people as the current cost of around $50 does.