This is different than having your price fluctuate just based on what material they choose, that a given in Allmoxy!
You can make your product fluctuate with the cost of a supply as it goes up and down for you, but the system doesn't automatically do it - many people do not want their pricing to change each time a supply (like lumber) changes a bit. You would need to create a variable for the cost of the supply - usually done by adding a column to a drop down attribute:
Then, for example, when one of those wood types changes in cost, you can come change it here, which will affect your formula.
