There is no cost (called cover charge) to enter. But it is usually expected of customers to at least order a drink.
Just so happens I was interviewed by the Washington Post. Here's the article from the Weekend Section February 11, 2005. Here's an excerpt of what the reporter Fritz Hahn gives about the show:
My flamenco group consists of one singer, one dancer and myself on guitar. We do two twenty-minute shows. The first at 8:00pm and the second at 9:00pm.