"when Santini makes his first performance, there is one thing which always bothers me. He takes a red scarf/bandana/napkin (I don't know the proper English word),
and suddenly some jar with flowers appears on the table. So for us as a viewers it is a nice tricks. But what about the people who sat
behind him and could clearly see how he makes his trick?"

If you watch that scene closely, the lady to the right of the picture is clearly holding the vase of flowers below the table. For a very brief moment you just catch a glimpse of it.

When Santini waves the cloth and holds it over the centre of the table you can clearly see the lady lift the vase up and place it behind the cloth. Watch her eyes and head move as she does so and see her arm change position.

You are right though, anyone looking from any angle other than ahead would easily spot how that trick was done either without the female assistant.

My question is this:
How can he make so much money at a club that seats less than one hundred?

I guess he's supposed to be world renowned. The David Copperfield of his day therefore he makes the real money on his tours and in bigger stages.

A bit like big bands or comedians who still perform warm up gigs at smaller, more intimate venues.

That would be my guess.