Guild Wars 2

I remember when I was farming TA dungeon back in late 2012 I made nice money selling Onyx Cores that used to drop there a lot. I took few months long break and when I came back, prices were down at few silvers, pretty much at same price like today.

Why cores and lodestones dropped in price so much since January ’13 (excluding charged ones)?

Because all that stuff now randomly drops from the Champ Bags that were evented somewhere in the last summer.

Yep, too much source from which you can get these cores/lodestones. The only that still have a high price is Charged because its the only one with a lot of use. A lot of cool looking weapons need charged lodestone, while for all the other lodestones you only have a couple of weapons that need them.