I've run into a strange cart addition problem. The cart adds the dollar value of the last option that adds cost to all non-null attributes below.
Let's say you have a item with a zero-dollar base cost.
Your first attribute is used to set the cost:
Two additional attributes have several zero-dollar attributes but are required:
When you select small-purple-striped and add to cart. The value in your cart is not $5 but $45. It adds $20 for each of the two attributes in the subsequent groups. If the second and third attributes are not required, it adds this amount to the total cost only for non-null attributes. So small-null-null is $5 but small-null-striped is $25.
For the product I've discovered this with, I've created a workaround by adding a $0 option item last to the attribute that sets the cost and then hiding that option--but this is far from ideal.