Patpong, Soi Cowboy, and Nana Plaza are notorious for ping-pong show scams. Quoted 'free' shows ramp into hundreds of dollars of forced drinks.
Bangkok's nightlife has plenty of legitimate options — rooftop bars, craft beer, jazz clubs, dance clubs. Stay out of the touts' orbit.
If you do enter a girly bar, ask prices upfront and only order one drink at a time.