One of the valuable functions of bargaining is that it tends to level out economic disparities. The seller adjusts the price to the ability of the buyer to pay. To people who are used to bargaining, having one inflexible price seems patently unfair. Your counter offer will depend on what an item is worth to you, and on how much the seller has asked. Sometimes a seller will ask a ridiculously high price just to test you.
Some tourists have been known to pay such a price, so occasionally it's worth a try.
In most situations, however, come back with an offer that is 30 to 50 percent of the first asking price, and go from there. If the seller flatly states that prices are fixed, try somewhere else, especially if it is a tourist-oriented shop. If it's a shop where others seem to be paying fixed prices, such as a chemist or camera shop, still ask for a 10 percent discount. You will usually get it. Buying more than one item, of course, will help you get a better deal.