See Odoo's documentation regarding Subcontracting, especially the section Inventory Valuation

Add landed cost ,50 € eaual qty to the transfer no of  route  receiving product from subcontractor

Create a subcontracting  BOM of your product A consume 100g(0.1kg)@

Costing method AVCO then made a purchase component  1kg @10€  AVCO now is 1€=0.1kg

Create a new service product (Subcontract Service) and select it as landed costed product , you also need to  create two Product Catalog ,one for each and use same stock valuation account.

Create normal purchase order on product A and it will create a subcontracting production and a receive route to inventory stock from subcontractor . Receive this stock and remember the input stock reference no . In Inventory , create a landed cost journal , the transfer reference no, valuation account , 50 €, and confirm it .

Your stock valuation report should be corrected 

You can follow this as reference and ignore the create new production location

https://www.odoo.com/sk_SK/forum/pomoc-1/v14-accounting-for-inventory-sent-to-subcontractor-as-part-of-cogs-183612 

Also watch the tutorial on Youtube, thank you ODOO  Sales Engineer   Kevin Zaki, he made a lot of wonderful tutorial for us.

https://www.youtube.com/watch?v=oMhZfUfWlUk

https://www.youtube.com/watch?v=biEvjW8cOU0