pay on (something)
1. To pay some amount of money that goes toward the total cost of something, especially when payment is recurring and ongoing. A noun or pronoun can be used between "pay" and "on" to specify or describe the amount that is being paid. He's paying more on his car each month than he is on groceries. I was tired of paying on a phone bill every month, so I switch to a pay-as-you-go plan.
2. To pay some amount of tax as a result of one's income. The amount of income tax I'm paying on my salary each month is nearly more than I take home with me! I'm going to have to pay nearly $3,000 in inheritance tax on my father's estate.
pay on something
to make a payment against a bill. You have to pay on this every month or we will repossess it. How much do you plan to pay on the car per month?