Hi,

To get the cost price of the product in the order lines if the vendor and his price is not set for the products.

Navigate to purchase -> models -> purchase.py

Then search the function named _onchange_quantity , ie, def _onchange_quantity , in this add an extra line

if not seller:
    self.price_unit = self.product_id.standard_price #add this line
    return

You can override this method to achieve this

Thanks