Not to state the obvious here, but if people can sell Alexandrite for 20k each then apparently some buyers do value Mythics at 600mil.
Here is why the price increases:
2 hours in Dynamis = 2mil
1.5 hours in Salvage = ~80 Alexandrite
(2000k/2 hrs) * (1.5 hrs/80 Alex) = 18.75k/Alex
So Alexandrite prices need to be around 15-20k each in order to be a competitive gil-farming alternative to Dynamis currency. Consider that it's more difficult to enter Salvage than Dynamis (3 person req, 500AP) and the lack of other benefits (gear is outdated, no AF2+2 xp) and you can see why the price is settling in the upper end of that range.
SE really needs to lower the requirements on these. Not only do you have to get 30,000 of these but you have to do everything else as well for items that, in most cases, are not as powerful as their empyreans. Not to mention this is the only useful relic type option for WHM. 300 MIL for the additional effect on this is a bit much seeing as current bazaar prices are 10k-12k. SE needs to do more to fix this than make ZNM drops 100%.
Here's the thing - the difficulty of a Mythic Weapon is now much higher than Square-Enix originally intended.
The proof of that is simple: when SE made these weapons they assumed most people would be doing Salvage, and thus there would be market supply of Alexandrite. This is true for Relic and Empyrean as well. Dynamis is still being done by people and Voidwatch also is the main event of the game at the time of this writing. This means that there is a steady market supply of Ancient Currency and Heavy Metal Plates/Riftdross/Riftcinder.
Salvage gear is outdated, and undesired for the most part. This is why Alexandrite supply is minimal. SE really needs to update Salvage gear if they want to return Mythics to their intended creation rate.
I actually like the difficulty it takes to create a mythic. Everyone's carrying an emyprean or relic weapon now. You know someone put in serious work when you see them holding a Mythic though. And you know they must like that job a lot to pick that weapon.
While the 15k per alexandrite price tag might seem a bit high, the main problem at this time is supply. I personally have no issue paying 15k each but the lack of availability is annoying at best. To the ones who say however the prices need to be high to be an alternative to dynamis I would like to point out that you can do it AND dynamis as well as the fact that you dont have to compete for mobs against 30 other players.
For those interested in extra gil and tired of dynamis or just want something more than dynamis to make money from grab 1 or 2 people/mules and do an SSR I promise these will sell faster than any dynamis currency you have ever sold ^-^ (best salvage I found is SSR archaic farm nets about 30-35 singles then you get 4 100% pouches from the HB PK GG3 CC route thena chance at linen from boss for an average not counting boss pouch of about 90-100 per run or 125 if you do count boss pouch)