You can't really though - e.g. 25 years ago you could fly economy from Vancouver Canada to Sydney Australia for $2600. (Inflation adjusted.) You can now do that flight for $1400. Pretty good, but the kicker is that flying business class is now $5300. Even premium economy is $4400. (Prices are all the cheapest I see on Google Flights over the rest of 2024, so some of them are limited to very specific dates.)

Probably the most equivalent price to 25 years ago is basic economy plus paying for a meal, snack, checked bag and selection of an exit row seat, but today's premium economy is probably the most equivalent service level to yesterday's basic economy.