不同的动词会有不同的介词搭配.
他花20块钱买了这本书.
He bought the book for 20 yuan.
He spent 20 yuan on the book.
He spent 20 yuan (in ) buying the book.
He bought the book at the price of 20 yuan.
有时也可以不用介词.The book cost him 20 yuan.
It took him 20 yuan to buy the book.
It took 20 yuan for him to buy the book.