Hi,
I’m not a teacher but i would just treat it like any professional transaction like a visit to the doctor’s, a lawyer or accountant.If you want to tip, then feel free to tip.Though to be honest i don’t think yoga and money really mixes very well.Though that is just my feeling on it and i guess people can choose to make a living somehow.The thought of commerce and yoga just sits funny here somehow though, sure, that is’nt what you asked.
It is more of an energetic and “spiritual” exchange between student/teacher/guru as Gordon stated.If you want to express your gratitiude for this then you can do it any way you like.It is the spirit behind any intention you may or may not have that counts.
As a common-place practice I’ve never heard of it but in a one-to-one context that’s a bit different.
In the past the student traditionally lived with the guru/teacher and took care of some of his/her welfare at least as well as household duties i believe while teachings were handed out piece-meal over a period of time, indeed typically a number of years. It’s not the same now and different culture and money is recognised currency today.
A book sounds like a better approach because they might just be a bit embarassed.Bear in mind i’m not a yoga teacher but I’m sure money is as attractive to them as anyone else.