It's positioned as a free tour, and you pay what you feel is appropriate.
We met our tour guide Anna at the Blanche Metro stop. Why would it be named White when it's in the middle of the red light district? Therein lies a story. French authorities, terrified by the great fire of London, made it obligatory to fireproof Paris dwellings, and this was done by applying plaster. The closest source of gypsum to make this plaster was Montmartre. The association led to the expression Plaster of Paris. When labourers arrived at the bottom of the hill with their wheelbarrows full of gypsum, they spilled so much the ground become white - hence the name Blanche. By the way, the hill of Montmartre was so honeycombed with holes from this quarrying that pillars had to be drilled down into the hill to support the Sacre Coeur church at the top.
Our tour wended its way from the bottom of the hill, up to the top of Montmartre, at times a fairly stiff walk. There were lovely views along the winding streets of the Sacre Coeur above us.
Along the way, we saw many famous places - the restaurant Aux 2 Moulins that appeared in the movie Amelie (we had happened there to eat lunch and I wouldn't recommend the food); places where varrious artists lived, and several places that these artists had painted. Annna would hold up pictures of the paintings so we could look at the paintings and the real thing at the same time.
Very good tour.