What is the normal fair to give to a thai language teacher

[playnice]

Hello everyone 

I have someone who posted that he do private or group thai course 

I asked him how much he said its up to me .. he even said i can treat him to a lunch in a restaurant,  doesn't have to be fancy

 

Well my question is how much should I pay him excluding the transportation fee.. i want to be fair to him and myself 

 

Thanks for passing by