Me and my wife went to a Ping Pong show in Thailand and got ripped off ...?

We went to Patpong road nigh markets, and just for a look went into one of the many bars that said "Free Entry and 100 Bhat beers", when we went in and sat down 10 minutes later they came with a bill for 2600 Bhat and a heap of bouncers. Now If they had just said 2000Bhat entry in the first place I would have gone in paid it and bought a few drinks ..... is there anyway I could have not got ripped off ? This has happend to lots of people in places from China - Prauge .. Is there a way to avoid it ? Please spare me "don't go into sleazy clubs'