我们先来看看来外对讨价还价这个概念是怎么理解的。

It is not a common practice in Western countries... I have a lot of problems coming to terms with this concept here. It is where you and the seller come from different ends of the price to pay for something. Obviously you want it as cheap as you can and the seller wants to sell it for as much money as they can. It then is a process of negotiating a price that both buyer and seller are willing to accept.

看来讨价还价在西方国家不是很常见的呢，但是我们还是要掌握怎么表达。
常见的有: negotiate a reduced price, beat sb down, bargain, haggle…

beat sb down: to persuade someone to reduce a price 说服某人降价，杀价

He wanted $8 000 for the car but I beat him down to $6 000.
他那辆汽车要价8000元，但我压到了6000元。

Bargain: to discuss the conditions of a sale, agreement etc, for example to try and get a lower price 讨价还价，商讨条件

He said he wasn’t prepared to bargain.
他说他不愿讨价还价。

Haggle: to argue when you are trying to agree about the price of something 讲价
通常搭配是: haggle (with sb) (over sth)

I left him in the market haggling over the price of a shirt.
我扔下他自己在市场上就一件衬衫讨价还价。