Noone knows how to make serpentstone anymore, and its only found in the old ruins belonging to the Serpentfolk (which in many cases is more ancient than the Azlanti society).
How much its worth? This is just a wild guess.
Not much in general i think, but to specific collectors it might be worth a couple 1000 gold i guess (although finding said interested parties could be an adventure in itself).
Revive, I am looking into this at present... appearently it can be used for major image spell but even in the paizo site a price isnt listed... so I am trying to figure out what/how/otherwise it works with weapons and armor since its listed as an optional thing.
I found I was directed to this:
((Magically Treated Walls:
These walls are stronger than
average, with a greater hardness, more hit points, and a
higher break DC. Magic can usually double the hardness
and hit points of a wall and add up to 20 to the break DC.
A magically treated wall also gains a saving throw against
spells that could affect it, with the save bonus equaling 2
+ 1/2 the caster level of the magic reinforcing the wall.
Creating a magic wall requires the Craft Wondrous Item
feat and the expenditure of 1,500 gp for each 10-foot-by-10-
foot wall section.))
so if that is re calculated somehow.... one could determine the cost of a dagger
I looked at the d20pfsrd site for serpenstone as I don't have the module.
The description under special materials didn't exactly say it is major image.
Instead the description says-
"serpentstone can be set to radiate heat or cold, or produce illusory smoke, roiling forth thin streams of scented, incense-like fumes designed to delight, enlighten, or otherwise inform serpentfolk in the vicinity. All of these effects function as major image (caster level 18th"
IOW, it gives those effects and they are treated like major image, caster level 18th.
Although there is no exact pricing in the description, the parameters (3rd level spell, caster level 18, at will?), can be used with the making magic items guidelines to determine a price for adding that to a weapon.
Note 1. From ultimate combat-
"Weapons Light and one-handed bludgeoning weapons, spears, axes, daggers and arrowheads can all be made of stone. Weapons made of stone have half the hardness of their base weapons, and have the fragile quality."
Note 2. Magic enhancements increase the hardness of weapons and there may be other methods of removing the fragile quality.
Note 3. Masterwork versions don't have the fragile quality unless the material does. So, in this case, they still would.
Note 4. The Reinforce armaments spell (1st level) removes the fragile quality, so it would be a matter of including that in the price of the weapon as an add-on like the spell-effect.