I've opened my town to turnip trading twice now, and it's really interesting to see how much/how little people tip! Some are super generous and some tell you they are done and to end while still at retail so they don't tip at all!
When I start turnip selling I'll be doing 99k per trip, so it's easy to keep a rack of, and you usually make 1mil+ each hand so really I feel that is an easy way to keep a track of how much to give, and I know personally how nerve racking and time consuming opening your town for turnip trading is!
I don't feel the need to use turnips for making bells anymore however, when I did use it, I would tip around 20% of the bells I had made. It's them using some of their own time to help you after all.