I've been wondering about that too. Maybe I'm stupid but it's kinda hard for me to imagine that as a magician, I could bring in enough customers that wouldn't have been there before to cover my multi-hundred dollar (for like a weekend maybe) fee AND still make the resteraunt a higher profit.
I can and have charged around from 300- 700$ for one hour of walk-around at an event.
However, with restaurants you aren't charging multi-hundred dollar fees. 200$ max probably per night, but I think that's even pushing it.
Usually with restaurants, since it is an ongoing gig, you would charge less. In my experience, I have made from 50-100$ per hour. Usually closer to the 50$ mark at 2-3 hours per night [which is per week].
So 100$ for a restaurant to shell out per week on one night of entertainment isn't that bad. Some of it is just purely for customer relations, but often you do get repeat customers or customers who come in on the specific night the magician is advertised for. Often, restaurants bring a magician in on the slow night of the week, to bring in customers.
Not every restaurant has a magician, some see it as a good investment, others do not.
Just my two cents.