I'm having quite a hard time seeing what the problem is here, but maybe I'm missing something... Based on Harald's analysis the projected annual shortfall is in the region of $350,000 per annum. Assuming ~5,000 attendees per annum, that equates to ~$70 per year per attendee.
The trouble with this analysis is that the 5000 attendees each year are not all different people. Many, if not most, people attend more than one IETF meeting per year.
A more accurate analysis would be: A shortfall of $350,000 per annum means ~$120,000 per IETF meeting. So, if 1200 people attend each IETF meeting, then that means $120 extra per person. (Or, if 2400 people attend each IETF meeting, then that means $60 extra per person.)
(Personally, I think that even the current $425 fee feels excessive, especially in the current economic climate.)
Ross.