As most of you reading this know, a supporting membership means you get the convention's publications (progress reports pre-con; souvenir book and pocket program post-con; some conventions also give you a membership badge even though you didn't attend), and the right to nominate and vote on the Hugo Awards, as well as the right to vote on the site of the N-years-hence (subject to payment of the Advance Supporting Membership fee for that Worldcon.
So what are the variable costs of a Worldcon supporting membership? I think they are the following. Figures shown are the 2002 cost per member of providing that item, based on 4100 pre-registered members, 6000 total members, and 715 supporting and no-show attending members to whom publications had to be mailed post-con.
Pre-con Publications, printing: $6.15
Pre-con Publications, postage: $2.09
At-con Publications (Souvenir Book and Pocket Program), net of advertising revenue: $5.24
Mailing post-con: $6.54
Membership Badge: $1.98
Total variable cost per supporting member: $22.00
It's only a cooincidence that the figures came out evenly, by the way. I did not fiddle with the individual entries to make it round off like that.
These are average costs. The postage costs, for instance, are significantly more for non-US members than US members, thanks to the low cost of non-profit bulk rate mail. This is why even Worldcons held outside of the USA may well print and mail their publications from the USA.
Obviously, individual Worldcons' figures will vary. ConJose's advertising revenue was lower than most Worldcons. Worldcons with larger non-North American memberships will have higher mailing costs even if they mail from within the USA. But also note that, with a reduction of Worldcon lead time to two years, pre-con publication costs should presumably drop by as much as one third.
This leads me to conclude that supporting memberships really are supporting Worldcons; that is, they contribute more than their own variable cost to running the convention, and anything that helps pay some of the overhead -- most Worldcon operating costs are fixed, not variable -- is a Good Thing.