Can I buy it cheaper?
可以便宜一點嗎？

I really want it, but don’t have that much money. Bring it down a little bit.
我真的很想要，但不夠錢買，可否降低一點價錢。

What price can you offer?
你可以給什麼好價錢？

What’s the best price?
最優惠價錢是多少？