I think that it depends. You should think about a couple of things:
1. Was it entertaining, interesting, informative and understable?
2. How is the economical level in your country(not the same Spain than Canada, for example)
3. How is the economical level in the country that you're visiting(obviously in Austria things are more expensive than in Poland, for example).
My experience as a Free Tour guide(it's already my job), people give between 5-8 euros per person. Sometimes you find people that give you 20€ per person, and other than only 20 cents of Euro per person. But usually, between 5-8 each one.
I hope that my answer helped you!!!