You need 2 Pyrite ore to create 1 Pyrium bar.
You need 3 Pyrium bars to create 1 Truegold.
The problem isnt that tho, its that they added Hardened Elementium to counter the Elementium dropping to insanely low prices on the 'normal ore' but the Pyrite ore isnt used that much.
Even the 4.2 craftables focus on Truegold, Hardened Elementium and Chaos Orbs.
Not Pyrium.
So its not worth to prospect now but its not really usefull for anything else, so the supply exceeds the demand.
And thus people stockpile it to prospect since its cheap