Plenty of people decide whether to lock BNB by staring only at "how much this airdrop might pay," and forget to subtract what locking costs them. Opportunity cost is just this: for the days that BNB sits locked, you could have used it for something else that earns a steadier return (say, a flexible-savings rate), and you're giving that up to chase the airdrop. Work out the return you forgo, subtract it from your airdrop estimate, and what's left is the part of the lock that actually pays off. To think this through properly, read how to work out the opportunity cost of locking BNB; for the two kinds of lock-and-earn events, see the Megadrop guide and how to catch HODLer airdrops.
One caveat: this counts only the steady return you forgo. The biggest unknown with locking is really price swings — BNB moving up or down while it's locked, which this tool can't estimate for you. The airdrop value is only your guess, too, and new tokens can drop below their launch price. Weigh all of that before you decide to lock.