[译] 他花了30元钱买了这些书。
[误] He bought these books at 30 yuan.

[正] He bought these books for 30 yuan.
[注] 表示“购买某些东西一共花去多少钱”，应该说 buy certain things for...，只有在表示“所购之物中每件的单价”时才说 buy sth. at... a piece. 再如:He bought these books at 30 yuan a piece. 他以每本 30 元买了这些书。