The dual pricing system in Thailand really annoys me. I have always refused to pay the "foreign" price for anything during the thirteen years I've been here. We don't have to pay different prices for petrol, or beer, or entry to the cinema so why should zoos, museums, tuk-tuks and taxis be any different? I wouldn't mind if it was means tested, there are a lot of very poor Thais but there are also a lot of very rich ones. It's racism, pure and simple. And they don't try and hide it. I get quite irrate when certain places try and charge me more simply because I'm a "Westerner". I usually find a few choice words in Thai does the trick. ;) It also helps that I can read the script, when the local prices are only displayed in Thai.
I guess it's no different to anywhere else but this is something you should be aware of if you plan to visit. But don't let it put you off.
My father in law is a taxi driver in Bangkok and he doesn't rip off anyone.