I just watched this documentary about ancient Athens and the narrator mentioned the Parthenon would have cost the equivalent of $100 million. How did they get to this number?
I get how they measure inflation and what a dollar today could buy, say, 100 years ago and vice versa. But this is based on things like CPI, unit outputs, etc. (my finance and economics background coming in handy!). Is it just the same process in this case, just taken back 25 centuries? Or is there something completely different i am missing?
Side question: What are some other interesting monetary comparisons like this?
You probably already realise this, but the equation makes absolutely no sense at all. You simply can't translate ancient costs into modern quantities of money and expect the result to make any sense at all.
The most frequently quoted estimate for the cost of the Parthenon comes from a 1953 article by R. S. Stanier, who put the building's cost at 469 talents (of silver). On the assumption that this is the estimate your documentary makers used (other estimates range from 400 talents to over 2000), the most likely candidate for the procedure they followed is:
Presumably minimum wage was a bit lower when the documentary was made, or maybe they estimated based on 7.5-hour days, or whatever. As I said, this is entirely nonsense anyway.
Just to show how nonsensical it is: you could just as easily convert 469 talents of silver to US dollars simply by calculating on the basis of today's silver price, which is USD$692.21 per kg, and at the rate of 1 Attic talent = 25.8 kg this comes out to a total cost of USD$8.4 million.
(You could add on the cost of the 40 talents of gold layered on the statue of Athena Parthenos and bring it up to over $50 million, but it's still pretty silly -- if we included the statue and then followed the daily-wage-based procedure, we'd end up with a total in the billions.)
Neither answer has any more validity than the other, and the fact that they're well over an order of magnitude apart should illustrate just how meaningless they both are.