Bell Borca notes mana values only if it's on the battlefield as a card is put into exile.

You'll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.

If a card in exile has in its mana cost, X is considered to be 0.

You must follow the normal timing permissions and restrictions for the card exiled by Bell Borca's last ability. If it's a land, you can't play it unless you have land plays available.

If you don't play the exiled card, it remains exiled.

Look at the card in exile, not its previous zone, to determine its mana value.

While not on the battlefield, and before any cards are noted for it each turn, Bell Borca's power is 0.