Which preposition to use with ducket

for Occurrences 2%

Camels laden with marchandise: for the which we paid 2. duckets for euery camels lading, and for euery 10.

at Occurrences 1%

percels of Opium, which cost me two thousand and a hundreth duckets, euery ducket at foure shillings two pence.

of Occurrences 1%

The ducket of gold is worth 9 tangas and a halfe good money, and yet not stable in price, for that when the ships depart from Goa to Cochin, they pay them at 9 tangas and 3 fourth partes, and 10 tangas, and that is the most that they are woorth.

Which preposition to use with  ducket