Why do you think this is like that?
Well, everyday life is expensive. Then you add air fare and hotel, car rental and meals to travel to meet teachers, seminar fees, etc., time in travel, missed work days, etc.. You need a pretty good cushion to absorb all that and the missed work.
Look at what Sagawa describes--traveling all over Japan with Takeda, and he had to pay for his own room and meals out of his own pockets. Mochizuki put everything into living a pure budo life. He was living with Morihei Ueshiba, doing sword and kobudo in the mornings, competing in judo shiai in the afternoons and doing aikido at night while living in an uninsulated dojo and having to watch out for O Sensei's needs. Finally, he got sick from so much hard breathing the moist winter weather of Tokyo and had to spend six months in the hospital.
I couldn't afford that. I don't know many people who could, and if they're teaching for a living, chances that they will have great health insurance are pretty dang low.
In Mochizuki's case, Jigoro Kano paid for his hospital stay since he was the one who had put Mochizuki on that incredible life.
After the hospital, Mochizuki went back to the milder weather of Shizuoka where his family built a dojo for him. So, again, plenty of money was available somewhere. Of course, Mochizuki at some point got a professional bone-setting license, so he did earn some money, but even so, the dojo was probably always a financial burden. If you ever work out how many students you would need and how much each of them would have to pay to cover the dojo, utilities, insurance and a living wage for the teacher, I can tell you, it's some ugly numbers.
What single-mindedly drove Sagawa?
His culture, in my opinion. Meiji roughneck, as someone said. He had the finances taken care of from the beginning, his father set him on the daito ryu path and he just determined to make that his ju-hachiban (18--a way of saying "your specialty").
As far as living such an obsession, when I first went to the old yoseikan in 1986, a Jietai Colonel asked me what I did for a living and I told him I was teaching yoseikan in my own dojo. He said, "Oh. Martial arts crazy. Yeah. I was like that when I was young. It drives your father crazy, I guess."
I said, "Yeah."
I guess it just depends on how far you commit to going with it.