There is a huge discusion around that on community list,

Yes it is not possible set cost on product.product only different price per variant.

Fabien's words only 1% of people need these feature because 99% manage same cost per variant.

COmpleting this it is only due to what is a "Variant" in Odoo a variant is simply a feature which don't increase "any" other property of the product.

Personally I think it is a huge problem of concept because industries where the variant is majorly used are basically the ones where the cost and physical properties changes impacting direclty the cost, but it is not as Odoo is designed.

You will need to create a product per cost (at least by default).

Regards.

PS: Don't forget say +1 if this answer works for you.