As someone who has been in kollel for many years and hopes to continue learning for life, I make sure to give the Rebbeim and Morahs a PERCENTAGE of the tuition I pay for that child.
When we use a taxi we give a 15 – 20 percent tip. Same when we tip the waiter at a restaurant we give a percentage of the total bill.
I am NOT suggesting a specific percentage but I think the Rebbe or Morah should get a percentage of the tuition paid – pick a FAIR and REASONABLE percentage, from as low as 5% (which is FAR lower than what you would give a taxi driver or a waiter!).
BTW – the Rebbe should ALSO be given the largest and nicest M’shlach Monos on Purim. This is mechanech your child that the Rebbe is the choshuvste person on your list. To your child, the Rebbe is more important than the local Rov or your Rosh Yeshiva/Kollel. It instills a chashivus for learning in your child!
I add Purim Gelt to the Rebbe and Morah on Purim too (as Mo’os Purim is mentioned in Shas – albeit in another context).