Vat/Tax Variant problem.

[elfis]

Hello

I have products with multiple size 5,10,20kg , and prices with Tax/Vat .
When I press "Calculate the Cost price" it calculate price as with Vat. How to do this for product variant ?
Product cost 10E for 5kg the 10kg is 5E more expensive and I want  to write 5E in Custom field or just 15E.
Please help.