there's no such thing as a free lunch

said to emphasize that you cannot get something for nothing

天下没有免费的午餐

A: I get to travel with my job but the downside is I have to give talks.

我这份工作可以经常去四处走走，不过坏处是我得经常作报告。

B: Well, there's no such thing as a free lunch.

哎呀，天下没有免费的午餐嘛。

A: This advertisement says I can get an expensive camera for free!

这个广告说我可以免费得到一台昂贵的相机!

B: Don't be so gullible. There's no such thing as a free lunch.

别那么容易上当。天下没有免费的午餐。