pay on ...用于pay duty on ...,表示“缴纳......的税”。如:
You have to pay duty on all electrical goods. 所有的电气产品都要纳税。(摘自《牛津英语搭配词典》第二版)
How much duty should I pay on the watch? 这块表我应该缴纳多少税?
pay for ...表示“付 ...... 的费用”。如:
How much did you pay for your new car ? 你花多少钱买的新车?(摘自《牛津英语搭配词典》第二版)