My personal observation is that seminars given by "experts" in other fields with, say, 40 years of experience in their field will mandate a much higher fee than that seen by pretty much any aikido seminar I've attended. If you take a look at the lecture fees that some professors from the academic realm charge, spending $100 for a weekend seminar with someone who studied with the founder is hardly anything compared.
In addition, I very much try to support as many seminars as I can. Of course, I want to learn from the teacher him/herself, but I also want to give support to these dojo who go through the trouble of inviting the teacher and putting on the entire seminar. If anything, it's a good way to help strengthen the aikido community, especially if it's a local seminar.
Just my thoughts,