Thank you all for your replies.
I understand that many senseis that live off Aikido actually do not earn very much compered with what they give (hours, weekends, away from home...).
I just thought that in some cases, when a seminar is intended to collect a certain amount of money, it would more easily be reached by lowering prices and getting more people to come than higher prices with less people.
Some seminars are for (example) only black belts, and that's fine- so they can get some close teachings without newbys. But since the highest ranking sensei here is 5th Dan, visiting sensei's are a sorce of new learning. Of course I can decide who I'd go train with and plan my year with what I can spend accordingly, but I can't lie that I would always be happy to attend as many seminars as I could, to learn more.
About having a group cashier- it's more of a problem. Aikido is still "young" here and groups can change and vary alot over the year. There is usually the basic group who are permanent trainers, and the others which some stay and some leave. So it's hard to supply for others because there are quite alot who also 'get by' (on their salary).
About a seminar being expensive or not- one should not only mention a price- it's greatly affected by the average income in that country (so a seminar may seem cheap but is average for the income of most people there).
I agree with all the different things you wrote in your comments, but as I said, I wanted to know what others thought of this subject. I don't want any sensei to become "cheap" in training and price, I just wonder if quantity will bring down the quality.