Réponse:   Hello from France ! 
Are you referring to the option "Add similar expense" from a sale document ("Ajouter une dépense similaire" depuis un document de vente)? If so, yes you are right, the system duplicates the content of the sale document on the expense creation form, which you can then modify. So the prices are by default the sell prices indicated on the invoice - the option works like the option "Add similar" ("Créer un doc. similaire").
I am forwarding your suggestion to see if (and when) what you propose could be implemented, as I believe it makes sense.  

Best regards, 
Justine.