Well, at first the idea behind Mythic and Relic was different. To get a Relic you needed to pretty much sponsor a Dynamis shell to get your currency. It was a time sink, but mostly a money sink.
The idea behind the Mythic Weapons was to have relic level weapons, but without the money sink. More missions that made it a much bigger time sink, but not the money sink.
Stage 1 you to defeat major bosses in the new Aht Urhgan, do all the assault missions twice. Stage 3 required you to do numrous ZNM fights to get a Pandamonium Warden pop-set, then use that set to get your key item for the final battle for your mythic. A mostly symbolic fight, but not a guaranteed win.
Stage 2 is where the big time sink came from. It required items bought with massive amounts of currency from Nyzul Isle, Salvage, and Einherjar. To get this it required months of Salvage runs, Nyzul Isle runs, and Einherjar (estimates have just the Einherjar stage at 6 months of tier 3 only runs). The Salvage bit was offset because it currency was sellable, and lots of people were doing salvage runs for the salvage equipment Alexandrite was pretty cheap. You were looking at ~30-90 million gil to flat out buy the alexandrite (1,000 - 3,000 a piece with a total of 30,000 needed). Unfortunately, Salvage equipment is now outdated, and no one does salvage, except for alexandrite. This has raised the price so just the alexandrite state is a money sink about the same as an entire relic weapon (current prices on most server come in at around 200-300 million gil if you were to buy all Alexandrite), yet still requires the extra time sink that Mythic weapons have had. You're doubly screwed.
