This report describes the affects of mixing grades of materials when crafting weapons. Adze's were crafted by a 125 skill weapon crafter with grade 1 and grade 2 materials.

So, it seems that metal and wood separately contribute to the bonus factor and these two factors accumulate. Iron adds 41 more to bonus than bronze and Ashe adds 8 more to bonus than oake. Thus Iron/Ashe has 49 more bonus than Bronze/Oake. The skill required to create an item is the skill of the highest grade material. It takes the same skill to craft an item with both components grade 2 as it takes to craft an item with one component grade 1 and one component grade 2. The difference in bonus seems to have no affect on the skill required to craft.