Ignore Federal Reserve Notes for a moment. That's just a unit of account and a very poor one at that. If everything was priced in BTC then how much would it cost to secure the network?
The same amount of electricity it uses to secure the network today. Not 2% of the world electricity.

The cost in true wealth, ie. food water, oil, electricity, homes, time, labor   will remain static or will grow as more miners come online.

Don't get hung up on the Federal Reserve Note price. That's a flawed way of using the word "wealth".   Federal Reserve notes are just the currency you are forced by law to use today, as a medium of exchange for actual wealth, babes, booze, and other material goods.

Try to think of it in term of "how much wealth, how much electricity as a percentage of the world total does this system use".

Do not conflate paper notes with wealth.
Have you read about Zimbabwe or the Weimar republic?  People there had trillion dollar notes but that wouldn't buy a loaf of bread.


What percentage of the worlds air, power, and people does it take to secure the network? That is the cost.
The amount of FRNs isn't an accurate way to measure this cost because FRNs lose value thru time as more and more are counterfeited into existence. If you dont understand what I'm saying then I highly encourage you to look into Weimar Germany and other hyperinflationary currencies.