It is possible to set up alternative Bill of material and Routing for a variant of an Item using the stock-keeping unit.

However - it is not possible to calculate standard cost for the variant.

In the inventory setup it is possible to use average cost calc type Item, or Item and location and Variant ?

There could be a difference in the cost - due to different bills og material or routing for the Item variants, but you are not able to calculate at this level.