Most customers don’t tip so don’t feel like it’s expected. They understand that training is expensive and is a big purchase for most.
There’s a lot of factors such as total cost of training, your income, # of weeks, whether or not the trainer was happy to send photos and updates if requested, and most importantly the temperament of the dog you brought in vs. what the trainer accomplished. A tough dog will have a good trainer pulling their hair out and doing everything they can accomplish and that deserves the same tip as them turning a cookie-cutter dog into a great hunter.
For the average dog with $1000+ in training and one months work: $50 is appreciated, $100 is average, $200 is exceptional, and $300-500 they will never forget that.