In my dojo there's a system when one becomes a shodan, one stop paying fees. In return, we help our sensei in teaching classes, running the dojo, etc.
When I wasn't a black belt, I was paying like U$3/month (yes, there are no missing zeros). This was to help the dojo to pay for its location and my sensei transportation costs. And we are allowed to go dojo hopping within the foundation where our dojo affiliated to, no extra fees/costs.
Teaching classes is not an obligation but it is an honor, at least for me that is. I may not be able to help the dojo financially, but I gladly contribute anything I can. This is my way of saying thanks to my sensei.
There is a shortage of yudansha here in Indonesia, I sometimes even teach in dojos that is not under my sensei. This is my way of saying thanks to the foundation for bringing Aikido into Indonesia. I also help the foundation in any way I can, especially during special events and kyu gradings.
The knowledge I got could never be measured in cash.