Item pricing (a potin and a rod)

[Tharkon]

I got the following two items, in two different campaigns and have trouble figuring out the items price

A potion that grants DR 3/-
Extrapolating from Heavy adamantine armour I would guess something like 150 gp, but that seems cheap.
I could not find any spell that grants DR/-, and I am not sure how long the potion lasts, but just wondering what level a spell would be that would grant DR 3/- and if at that level if should be 1 rnd/lvl or 1 min/lvl

A rod (it's called a wand ingame but it should have rod stats) that grants the following:
Greater Spell Focus (All schools)
Eschew Materials
The ability to ignore spell failure when it is 10% or lower
The rod can't be used as a weapon

 

[Tharkon]

I found the Psionatrix, which is an amulet that grants +1 to DC's of one psionic discipline for 8000gp. So this rod is gonna be something really expensive.

Then again, something that boosts Intelligence by 4 would also boost all saves by 2 and would only cost 16000gp.

 

[Tharkon]

A Greater Rod of Heighten Spell +1 would cost 24500gp.
Giving this rod infinite charges would multiply it by 5/3 for 40833 gp.
The spell failure thing is similar to the Twilight ability which is worth a +1 armor bonus, it's slightly less powerful so let's assume we can get it at it's cheapest price of 2000gp. So 42833 gp total now. The Eschew Materials feat isn't that super so just rounding up to 43000gp would probably do.