For most ot them, fifty pence is a small price to pay for a nice big bar of chocolate.

划线不定式作定语，修饰 a small price。被修饰语 a small price 是 pay 的逻辑宾语。for 表示交换关系。

pay a small price for a nice big bar of chocolate 

结构为：pay money for sth. 付钱买东西。