General questions and discussions.

Yes, adding a gold cost for transformation (a fixed price for converting each unit of resource would be the most logical) would look good and add to the atmosphere.

As another option, you could group the resources into pairs. Sulfur <-> Mercury, Crystals <-> Dragonstones.

You could also change these pairs each time they are generated. Then you could keep the exchange rate at 1:1 and not use gold.