No need, if they share a SKU.
When products have the same SKU on both platforms, a button appears in the top corner to sync costs: it automatically copies the costs you already entered to the products on the other platform.
After syncing
Download the Mercado Libre Excel file: it will come with the synced costs already filled in, and you only need to complete the missing ones. It's much faster than starting from scratch, because the file already has the work done.