You can't discard a card at random, see what it is, and then decide whether or not to pay the mana. Mana abilities must be activated before costs are paid. At the time you pay the cost of discarding a card, you either have enough mana to pay the , in which case you must continue no matter what you discarded, or you don't have enough mana to pay the , in which case you must back up no matter what you discarded.