Let's look at some of the most famous custom's duties implemented in North America - the Townshend Acts of 1767: 7 George III, c. 46 ; (available here) These were the duties imposed on North America following the failure and retraction of the hated Stamp act of 1765, which can be found transcribed here. The vast majority of items on this list were different styles and qualities of paper, but among the most famous was the tax on tea, which for every pound of tea imported, the tax was 3 pence.

As an aside, it's worth noting that on the Isle of Man (7 George 3, c, 45) the custom on a pound of tea imported from England was 1 shilling. Englishmen living on the Isle of Man paid a tax 4 times higher than any living in North America (12 pence to the shilling, 20 shillings to the pound).

The main authority on currency conversions from the first British Empire to modern values is John McCusker, who has written extensively on the subject ("Current Value of English Exports" WMQ, 1971, pp. 607-628; How Much is that in Real Money?). The ratio of value from 1767 to 1991 (date of McCusker's publication) is 95:1629 (or about 1:17). The problem is exacerbated because the value of sterling currency (what was stated in the statute) had different values in local currency. 100 pounds sterling was worth 131 in Massachusetts, but worth 697 in South Carolina. To try and accommodate for this, I'm going to use Virginia, Pennsylvania, and Massachusetts to give as best of an estimate as I can.

So, the tax was 0.0125 per pound. Here is the formula I followed to get a price in 1991 US Dollars

State: (Tax amount*(State Currency/Pounds sterling))(Ratio of 1767 to 1991) = Value in 1991 Currency

MA: (0.0125*(338.09/100))(17.147) = $0.725

PA: (0.0125*(274.45/100))(17.147) = $0.588

VA: (0.0125*(359.18/100))(17.147) = $0.770

Lastly, using this inflation calculator, here are the final taxable amounts on 1 lb of tea in 1767 converted to modern dollars:

MA: $1.28 per pound

PA: $1.04 per pound

VA: $1.36 per pound