I've found on wiki following description of tolls
"This advancement increases the tax rate levied on goods passing through the province of 5 points."
So if someone buy wood for 1000 pounds with tax rate 10%, he has to pay 100 pounds of taxes. What will happen if I have got tolls.
1. I'll increase the tax rate up to 15%, so the buyer will have to pay 150 pounds of taxes and I'll get 50 pounds.
2. I'll increase the tax rate up to 15%, so the buyer will have to pay 150 pounds of taxes but I'll get only 5% of 150 pounds - 7,5 pounds.
3. I'll get 5% of base tax value (100 pounds), so the buyer will have to pay 105 pounds of taxes and I'll get only 5 pounds.