I would not say with food vendors. street markets selling clothes and other good then yes you must or you will end up paying silly prices. It is expected.

I was once told come down to a third and then back up to a half. not always the case. The longer you are thee the more you will get used to it.

Things like DVDs are aound 100 each and 11 for a 1,000.

At the end of the day it is what it is worth to you. I find in the uK now that many of our retailers offer goods at cheaper than the price in Thailand.
Do your calculations on the exchange rate right as well I see many posts where people say things like "we got accommodation for 600 bhat, Thats just £5.00" it is more like £10.00 so it is easy to misjudge.
another tip would be to go to somewhere like tesco Lotus early in the holiday and see what the prices are like there it will give you an idea of the set prices.
I think that every one of us falls foul of paying over the odds at the begining of the holidays.
Make sure that taxis are metered and always agree a price.

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Kiss
God laughs at people who make plans.