There's 40 trillion USD worth of liquid fiat currency total (currency + checking accounts)
A 40 trillion market cap for bitcoin would mean that BTC = $2,500,000
This 40 trillion represents the whole market that bitcoin is "competing" in against fiat currency (and alt coins), and doesn't take into account BTC's use as a store of value--gold, for example, has a 4 trillion USD value. So if fiat currency became worthless, that's the bottom for BTC (bottom because it doesn't include potential market cap hikes from the derivatives market and institutional $ investments that are to come)