Inconsequenti-AL
Breaks Games
You could wish a non magical item of up to 25,000 GP. Although that's a hellishly expensive way to do it? 
I'd probably allow limited wish to transmute normal metal into mithral (or deep crystal). Although anything more than 1000gp worth and I'd probably throw an additional xp cost on top of it. Or insist the base metal was as valuable as the mithral.

I'd probably allow limited wish to transmute normal metal into mithral (or deep crystal). Although anything more than 1000gp worth and I'd probably throw an additional xp cost on top of it. Or insist the base metal was as valuable as the mithral.