I hesitated on giving business 101 advice, not being as educated as some others. but i think you can use some basic business guidelines. i believe senseis should be compensated as well as possible, even if they are volunteers they deserve gifts from us.
think of a seminar as a business:
if you sell out - you charged too little.
if you don't have a good turnout - you charged too much OR you didn't advertise.
identify your market, and use past performance to determine how you should
set prices for the future. keep records. use free software online to analyze your data.
if i tell you what you already know i apologize. and i in no way suggest that aikido training be watered down to make it appeal to the masses. judo was more fun before that kind of mistake was made.