Tipping is a very personal think Alan and much depends on how pleased you were with the service given. We always used to club together and give 20 levs each. The only think I would add is don't give them gifts, that doesn't feed their families. They are on a pretty low income as instructors and they do rely on the tips to get them through.
Good to see you and the family are coming back to Boro. I agree with Pete that it's very personal. Some people don't give anything, some give 20lev and some people have given 100lev. It's really down to the service you get...
The instructors are on a low wage and the season is relatively short. They do depend on tips to make up their wages, but dont feel obliged if you weren't happy with the service