Solo as in one person, not one account. You need 3 to enter, so even if you can do it with one player you still can't "solo" salvage with only one account. You either use friends or mules to enter yourself. It actually used to be possible to force a slow DC and enter yourself with only two accounts as long as you had another mule on the same account with Salvage access, but it's hard.

Anyway, if you kill all the archaic machinery, get the 4 pouches, and kill boss with TH3+ you will pull about 100 Alexandrite per run on average. If you do it with TH2, maybe closer to 90. I find that going DNC/THF with a healer mule pushes me through the zone in 60 minutes and makes better gil/hr than my THF even though my THF gets more drops. BLU/RDM can similarly solo the entire zone with TH2 without even sacrificing their subjob (just DoT the boss with Dissevement or something).

If you count getting it to 99, Mythics are about 3-4 times as expensive as relics at the moment. However, you have to remember that relics have become substantially easier to get since 75 and Mythics have become only marginally easier to get. Regardless which one you're talking about, they're both easier to get now than they ever have been before.