I think, 5 euros per person sounds alright. Please note, Anna is a licensed tour guide in Vienna and becoming an official tour guide is not easy. Usually, the group contains 10/15 people, so 5 euros per person makes sense. But if the group is too small, consider paying 10 euros.

Basically everybody who can afford it gives something.

Considering what another guided tour costs, we give around 10-20€/person

I think one can not answer this question without having done the tour. I see it like a rating system. I would give 5€ only for quite bad tours (but in that case I probably would have left the tour at the beginning) and 10-20€ or more for a well done tour. You should show your appreciation if you liked it, and not do this tours to save some money that you spend elsewhere. Pay what you think it is worth and of course what fits your budget. There is no good answer to this question, it is for you to decide, that is what makes these tours so great!

As a student I generally do between 10 and 15 for “free” walking tours

As a student I generally do between 10 and 15 for “free” walking tours