What is the price they are charging and what are you getting for it? Do they pick you up from your hotel, use a van, or do you meet them somewhere. How many hours have you booked or are the number of hours set? Does the tour include entry into any buildings or museums with a fee? Will you stop for snacks, or meals?
How much to tip does also depend on how well you liked the tour. Did it match your expectations, did you enjoy it, or was it boring? Do you feel like you got value for your money, and that the guide knew what they were talking about and did they present it in an enjoyable fashion? Were they passionate about what they were doing, or did they seem bored and were just repeating a memorized script? Have you asked for specific themes to be covered and did they do so? Sharing emails with them and letting them know your interests and likes, will help them plan your tour.
If everything was super, and you felt they went above and beyond to make your tour memorable, then tip 10-15%. Buy them lunch or a drink if you are happy with your tour. If you thought the tour was crappy, don't tip anything at all. If it was just average, then reduce the tip, but not lower than 5 euro. Better to tip nothing at all than to tip 2 euro on a 200 euro tour.