By default, your cost price on a sales order will show the product or preferred catalog cost price initially but will write back the accurate cost once the item has been delivered from the linked PO cost or the allocated batch cost from stock.
If you do not want your landed cost prices to overwrite the SO cost from the PO or allocated batch price used once delivered, you can change this in admin in system keys>movements system keys:
You can also change when the cost prices write back from the linked PO or allocated batch price used in admin>system keys>purchase system keys:
