Many seminars are run on a business footing and are intended to keep the teacher in a reasonable lifestyle, so I can't see someone giving up what is in effect part of their salary in order to provide someone else with a cheap invite. Some people are in ma for their own edification, others are trying to make a living at it, hence the differing costs - which I agree can sometimes be inflated, but I've never been impressed by the argument that everyone should teach solely for the "love of the art". I like to have a sprinkling of true professionals available as well as the gifted hobbyist.
Couldn't your dojo instead set up some sort of joint fund so that everyone contributes so much a week/month and you have a rotating list of people who attend the more expensive seminars. The only people who would potentially lose out are those who drop out of training, so no real loss methinks?