Metamagic Rods of Elemental Substitution?

[SteelDraco]

Has anyone considered a way to price these items? I've pondered just using the price of the cheapest normal rods. Thoughts?

 

[Pyrex]

In Tome & Blood the +0 rods were priced as half the cost of the +1 rods. If you're willing to assume that's still reasonable, a lesser rod of E. Sub. should be 1500gp.

 

[Terath Ninir]

I'd make sonic substitution cost as much as a +1 rod, though. That substitution should be +1 level in general, though. It's definitely more powerful than the other substituitions.