Tipping high-level instructor

How much are you guys tipping instructors on lessons? I've taken two two-hour advanced lessons with the top guy/lead instructor at my resort and didn't have any cash to tip him with, so I need to bring in something* next time I see him. The lessons were $300 each. This guy doesn't usually deliver many lessons himself-- just trains the other instructors and manages them.

*I just bought a gift for my instructor--a weather-proof pen and notebook--and was going to drop it off with some money. My question is what's the right amount of money to ensure I don't annoy or insult him? I was thinking $100 to cover tips from the last two lessons and the next one-- which would be $33/lesson and slightly more than 10%. Part of me feels like the lesson is expensive enough without a tip and the other part of me feels like I owe him something extra. FWIW I'm planning to take another 1-3 lessons this season with him and plan to attend his summer intensive camp; he's also helping me select and buy a new board with a Rossi rep he knows.