In D&D, by the core rules, there's no monetery conversion for Gold to Mithral, since Mithral isn't currency in D&D. The only pricing for Mithral, by the core rules, is the price adjustments for making weapons and armor made of Mithral in the DMG (that is, +1,000 gp for Light Armor, +4,000 for Medium Armor, +9,000 for Heavy Armor, +1,000 for Shields, and +500 per pound of other items). Mithral, however, isn't meant to be currency, and there really isn't a good way to convert it directly to money.