Is the minimum mana cost multiplied as well in Stone to Earth and other similar spells?

Question

In GURPS 4th Edition Magic, several spells have mana cost multipliers that apply in certain conditions. For example, the Stone to Earth spell has multiplier 2 for transforming metal to earth. The spell also has minimum mana cost 6. When transforming metal to earth, is the minumum cost still 6, or is it multiplied to 12?

Stone to Earth Regular

Turns any kind of stone (including gemstone) into simple earth. Must be cast on a whole stone or block, rather than a part of it. This spell also transmutes metal into stone, or transmutes metal into earth for double cost.

Duration: Permanent.
Cost: 6 per cubic yard (minimum 6).
Prerequisites: Earth to Stone or any four Earth spells.

Other Earth spells that also have a minimum cost and multipliers are Earth to Stone and Shape Earth.

Answer 1

My interpretation is that the minimum cost for transforming metal to earth is 12. The minimum cost is multiplied by the same factor as the overall cost.

My reasons for this are that there's nothing said about the minimum cost being fixed, and for several of these spells, the minimum cost is described in terms of a minimum volume affected.