If you want to add an upcharge to a product you can go to the product attribute and add a column for price. In the first example below you can see that we added a "wood_price" column and entered in the price per each type of wood.
Secondly go to the product you would like to add a an upcharge to and enter the formula. i.e. in the second picture below you can see that the product we are editing is a thick door which costs $38.5. Simply enter "38.5+..." and select the product attribute from the right hand side which in this case is .wood_price.
