You usually woudn't ask for a lower price, But if it's an error on your bill, you can simply say: There's an error on my bill, it's too expensive/it's the wrong price. If you want to negociate your price on something you can propose your price. Ex: a bracelet is 15$ you could say I want it for 13$ or Can I pay 13$ for this? If you mean when your are searching ex: for furniture and you want the cheapest price. You could say I want the cheapest chair here. :) !

It depends where you are going if you will go to a big department store price tag is final but if you will go to a public market you can ask nicely if they can lower the price or make a negotiation.

i agree with all the answers above.

Lea-chan has brought up a good point: in a store, the price is usually fixed, so asking for a discount on a single item makes you look very rude. However, if you're doing business with a supplier then it's acceptable. If you're buying in bulk, or buying a number of items, then there's a tiny possibility. If you want to negotiate, then do it in the most diplomatic way possible. "Would any discount be possible on that?" "Is it possible for you to give a discount?" In most cases, don't be surprised if the answer is "no". It's a cultural difference.