I don't wrench on cars for a living so, my perspective on this is likely to be quite different from those who make or augment their living with their tools, skills and labor. I'm in this for the fun of it. Long ago I thought the coolest job in the world would be working on hot rods but in recent years I fear that if I was doing it for a living I might lose my love for it and maybe even begin to hate it. Then what would I do for fun? Golf? I don't think so!
Except for a very small select group of friends who I know have the same perspective that I do, I always ask once "how much do I owe you?" after someone has done some work for me. It does not matter to me if they are in the business or moonlighting in their garage, I feel that is the considerate thing to do. I do not want to presume upon any relationship or take advantage of anyone. I'll pay whatever they think is fair for their effort. If they don't charge, I thank them and make mental note that this person has "favors on deposit" with me. I don't play the tit-for-tat game but I do go out of my way to help those who have helped me in the past.
For the aforementioned small select group, the rule with all of us is "I don't want to make money off my buds". The exception being, that if the one receiving the goods and/or services is doing it to make money, then the one providing the goods and/or services gets to make money on the deal too.
I offer different levels of help to different people based on their situation, closeness of the relationship and how much they've helped me in the past. For single moms who struggle to provide food and shelter for their kids I've bought parts and made repairs and would not accept any payment. If I see a rodder stopped on the side of the road, I will stop and offer help. I won't accept any payment there either. If I'm ever broken down on the side of the road I hope someone will do the same for me. That'll be my payment. Sort of like "hot rod Karma". For my family and 2 or 3 really close friends, I will drop everything to help them anytime anywhere because I know they won't abuse the offer and I know they will, and have, done the same for me. For anyone else I don't usually do work for them. I will help them and work right along side them as long as they want to work. Just a few rules to follow there. When they stop working, I stop working. They buy their own parts and supplies. They can't leave a dead car at my house. Once in a while I will do work for friends, like a tranny rebuild. I tell them up front that they have to pay for all the parts and I give them an idea what it's going to cost.
The "payment" that I receive is more valuable to me than money. Sometimes my "payment" is the good feeling of knowing that I helped someone who really needed help and appreciated it. Sometimes the "payment" is being able to thank, in a tangible way, someone who has helped me and other times the "payment" is just the good time that I had hanging out with my buds wrenching on old cars. It doesn't get any better than that. With all this payment I receive, I am a rich man.