Many years ago, I left an airline, and was liable for a training bond. I had fortunately been able to find out, accurately, all the costs related to that training, so when I was presented with a bill, I responded with a counter-offer, listing the total cost of my training to the company, and an appropriate amount. Obviously, the company rejected it, and so decided to sue me. They settled for my offer amount on the steps of the courts. My lawyer said that they are not allowed to make money out of you leaving, they can merely reclaim any costs incurred, so if you know the costs, you can make a realistic offer.
The above was 13 years ago, the law may have changed.