In "A History of Agriculture and Prices in England: 1583-1702" by james Edwin Thorold Rogers, pp738, one finds various references to gunpowder prices recorded by Samuel Pepys as:
1589: 8d. / pound
1642: 2s. / pound
average of 15 entries over the period:
14s. 3½d. / dozen pounds
£6 5s. / cwt
Specific prices for purchase by the barrel, for naval ...