We already spend a total of $15 million dollars per day in energy costs[1] making secure transactions in Bitcoin, so I think in a way we're already spending more than $5 billion on Bitcoin security.
I ran the numbers on newer mining rigs a few weeks ago and came up with a PH/s needing about 1 semi tractor trailer engine to run. The network is currently doing about 130PH/s, so that's about 130 semis engines running. Not anything to sneeze about, but also definitely not $15M a day.