They are free, tips are at your discretion (and I have nothing to gain or to do with the organisers!).
In my experience everything seems really expensive in Switzerland so it is a blessing to find some that is both free and worthwhile.
I can recommend the free walking tours of Zurich, having attended both the New in Zurich and the Downtown/Historical tour. The former are orientated at us Ausländers who are unfamiliar with quaint Swiss customs like wrapping up your old newspapers like Christmas presents for recycling
I went on this tour with my sister who visited me just before COVID. It was very enjoyable but note: tips are expected - not mandated mind you. But shame on those who do not tip after the tour (the majority did and then some!)
(sorry - some links in German)
Another source for free (self-guided) walking tours in Zürich are these great guides made by the City of Zürich that can be ordered for free or downloaded as pdfs.
Five are available in English:
01 - A walk through the Inner City
03 - A walk along the River Sihl
07 - A walk through the University District
22 - A walk with children
23 - A walk all around water
They also made mp3 radio plays for seven of them (I think only in German).
The maps are great... they suggest walking tours with points of interest not only for downtown, through different neighborhoods, some which you simply might not think might be super interesting --> one of my favorites was Schwamendingen.
A highlight... did you know there are two scale models of Zürich city that are free and open to the public (a historic one and one that is part of the zurich city planning department ).