EXPENSIVE
(TO BE)

guì
CHEAP
便宜
piányì

This really isn't cheap.
这个 真 不 便宜。
<Zhège zhēn bù piányi.>
Going by plane is faster but more expensive.
坐 飞机 快 可是 贵。
Zuò fēijī kuài kěshì guì.
It is too expensive. I don't want (it).
太 贵 了,我 不 要 了。
Tài guìle, wǒ bù yào le.
Is the fruits in China expensive?
在 中国 水果 贵 不 贵?
Zài Zhōngguó shuǐguǒ guì-bu-gui?
The more people are there, the more expensive the stuff is.
人 越 多 东西 越 贵。
Rén yuè duō dōngxi yuè guì.
The book that I bought yesterday is good but expensive.
我 昨天 买 的 书 很 好 可是 很 贵。
Wǒ zuótiān mǎi de shū hěn hǎo kěshì hěn guì.