Getting cheaper mangoes - just keep my mouth shut

My language teacher was sharing with me this week about a conversation she had at the market. She asked a lady selling fruit "Why does the sign say 20 baht per kilo?", because she knew that the price should be 15 baht per kilo. Oh, it's the price for the foreigners. My language teacher relayed his conversation with a hint of disgust.

Last week I'd gone to the market and asked in my best Thai, "how much a kilo" for the mangoes I wanted to buy. "60 baht", the seller replied. I thought this was a reasonable price as I heard my wife telling me only a couple of weeks ago that they were selling for 80 baht a kilo. This week I went to the same seller. This till I didn't open my mouth. I just picked out the mangoes I wanted and waited for the seller. She said 40 baht a kilo! So by not opening my mouth I saved myself over 30%! I guess I can pass for a Thai when I don't open my mouth!