BTC was created with a new transaction system without a central authority. The BTC can be used in the future as the main currency that we gonna pay with. But what's gonna be the value of a main currency? What's the value of dollars or eur?
-Answer is: The dollar IS the value. The dollar doesn't cost anything. The dollar is the value.
If BTC is gonna be used in future, it not gonna cost anything, because the BTC is gonna be the value. It not gonna be like Bitcoin cost 50,000dollars... Bitcoin not gonna cost anything because Bitcoin IS the value. But that's probably not gonna happen. We not gonna pay with BTC but with stable coins like USDT...
I think you are asking, if BTC were the only currency in existence in the world, what would it's purchasing power be? How can that be determined? The answer to that is this: the purchasing power of a currency is the quantity or number of that currency in existence divided by an accounting of all the physical wealth that exists. So in this case:
18,923,556 BTC / [total accounting of all physical wealth such as all real estate, all physical tangible resources, all valuable physical assets that exist] = the purchasing power of BTC
In this case the total accounting of all the world's wealth adds up to $360,603 billion US dollars.
18,923,556 BTC / $360,603,000,000,000 = 5.24x10^-8 BTC, or 5.24 satoshis, per $1.00
In other words, 5.24 satoshis would represent the wealth that 1 USD used to represent. So in the United States at current housing prices, you would need roughly 1,572,000 satoshis to buy a house.
Someone check on my math.