I don't think you're considering the intrinsic value of the chisels either. After all is said and done you'd be able to sell the chisel for about what you bought it for. That and even if the chisels only increased overall gem value by about 20% (after all losses), so you'd need 125m worth of gems to "double your investment", a person in the tier this is aimed at (read: likely capped, likely playing more than is normal) would probably take less than a year.
Essentially this becomes a lifetime transferable annuity based on how actively you gather treasure, the intrinsic value will always make the pricetag seem absurd.