If casting the revealed card involves paying a mandatory additional cost (such as the one Fling has), the player casting that card must pay that cost if able. If they can't, the card remains uncast in their hand. If the mandatory additional cost includes a mana payment, the situation is more complex. If the player has enough unspent mana to pay the cost, that player must do so. If the player can't possibly pay the cost, the card remains uncast in their hand. However, if the player has the means to produce enough mana to pay the cost, then they have a choice: The player may cast the card, produce mana, and pay the cost. Or the player may choose to activate no mana abilities, thus making the card impossible to cast because the additional mana can't be paid.