There is no hard and fast rule on this issue.
Much of it depends on what kind of day the Customs Inspector is having, and the attitude you have when you speak with him.
As a general guideline, if the inspectors assess duty for overages, it runs in the 10% to 15% range.
However, bear in mind that these people are American Government Employees who belong to a very strong union - and carry guns.
This is generally not a very good combination - IMO.
In any case, most of these folks are not too excited about filling out extra paperwork in order to collect just a few dollars in duty. If they are having a good day and you treat them with a bit of respect, in most cases they just wave you through and tell you to have a nice day.
There is an additional ploy that I have been using for years.
It is allowed to pay your additional duty with a credit card.
The Customs Officials especially do not want to have to put down their donuts to walk you to the credit card machine to pay what you owe.
Whenever I enter the USA with goods over the limit, I fill out the form honestly, have a big smile on my face, credit card visible in my hand, and find the nearest Officer to ask where I can pay.
In over 30 years of visiting the USA - very often with goods over the limit - I have never been charged anything.