The more people are there, the more expensive the stuff is.
人 越 多 东西 越 贵。
This really isn't cheap.
这个 真 不 便宜。
Going by plane is faster but more expensive.
坐 飞机 快 可是 贵。
This is more expensive than that.
这个 比 那个 贵。
This is 20kuai more expensive than that.
这个 比 那个 贵 20 块。
Could you give it to me a bit cheaper?
你 能 不能 给 我 便宜？