I'm not actually fond of upkeep costs, personally. They just seem a lazy way of balancing a card that ends up creating more problems than it cures. While having counters for it would work, somewhat, it still gets around the fact that late game, you can drop a whole bunch of these out at once, and within a turn or two burn through most of your foe's creatures.
What you could do with the upped instead is to turn it into a weapon, say:
5
Weapon: Deals 5 damage to opponent.
: Target flying creature loses airborne for 1 turn and takes 1 damage.