You pay what you pay.
If this month, you spend $1000 on materials that you use to make finished items that you sell for $2000, you have $1000 of profit. If next month, the materials are not on sale and you spend $1500 for the same materials, and sell the same finished items for $2000, you make a $500 profit.
You don't adjust your taxes for the fact that materials change cost. You pay what you pay.
