We have created a new UoM called Box500

We manage to associate this  UoM to a product called Paper Cup therefore every time we will receive in stock 1 Box500 the stock quantity increase by 500 unit ... so far so good.

The problem is that the cost for one box500 is €50.00 so we would like to see the stock value increase by  €50.00 (of course after receiving the item) but unfortunately the stock value increase only by €0.10 and €49.90 go to the purchasing price variant...

It seems that the stock value increase only by 1 cup value (box500 €50.00 / 500) rather than on box500 (€50.00) or 500 cups (Box 500 €50 / 500 = 0.10 one cup cost x 500 unit = €50) 

Could anyone please let us know how to fix this?