spend有两种用法：
1、主语 + spend + 时间/钱 + on something.
例：I spent 4 yuan on the book.
2、主语 + spend + 时间/钱 + in doing something(“in”可以省略）.
例：She spent 14 dollars (in) buying this pair of shoes.
pay有一种不同的用法：
pay + 钱 + for sth.
例：He paid 90 pounds for the chair.