Pete,
I have a friend in Borovets (prev ski instructor) and I was going to offer him cash to do some lessons. Happy to pay his ski pass and lunch bill but didn't want to insult him with a stupid price. Not sure what a days pay comes to in Bulgaria.
A lot will depend on how busy they are Craig and when you are going. If they are not too busy I'm sure you will get a 'deal' as they'd sooner be earning something rather than just hanging around.
As I've always said in business, 1% of something is better than 100% of nothing!