I've gotten totally different fares (couple hundred difference) on TK based on whether I booked through US or international page. They actually wouldn't bill me because "I wasn't supposed to be able to do that"...I argued and they finally said I could pay in person, luckily I live very close to LAX and would have time in Istanbul before flights and told them it wasn't a problem. Seeing this didn't dissuade me they called me back and ended up taking it over the phone from the office in Istanbul.