I find the phrase "very expensive" is alway a good thing to mutter in the local language before starting the negatiation. I always start with a ridiculously low price and work up without wasting too much time if they dont come near your perceived value. After about four bids I will start to slowly waqlk away as many people here suggest. You get the best price bid when you are about 10 meters away and you see them heading into their stall. Thats the time to buy or move on.