EXPENSIVE
(TO BE)

guì
CHEAP
便宜
piányì

That is expensive, this is slightly cheaper.
那个 很 贵,这个 便宜 一点。
Nàge hěn guì, zhège piányi yīdiǎn.
This is more expensive than that.
这个 比 那个 贵。
<Zhège bǐ nàge guì.>
This is 20kuai more expensive than that.
这个 比 那个 贵 20 块。
<Zhège bǐ nàge guì 20 kuài.>
This is 100 more expensive than that.
这个 比 那个 贵 一百 块。
Zhège bǐ nàge guì yībǎi kuài.
This really isn't cheap.
这个 真 不 便宜。
<Zhège zhēn bù piányi.>
It is too expensive. I don't want (it).
太 贵 了,我 不 要 了。
Tài guìle, wǒ bù yào le.