How much should this weapon cost?

[Stormthorn]

It has abilities not covered in the SRD, being homebrewed item.

Butterfly Death

This dark gray +2 Ghost Touch shortsword grants an additional +6 bonus to Hit and Damage for its users sneak attacks. In addition, any critical strike with the weapon inflicts 1d6 negetive levels on the target. If a being is reduced to level 0 or below by this weapon their body turns into a swarm of diminutive flying insects (butterflies). This weapon is pseudo-sentiant and will inflict a single point of force damage per round on any being with less than 12HD that attempts to wield it.

[Frog Dragon]

It price it around 50000gp if I had to give an exact sum I'd probably say 48000.

Seems to be right considering the normal prices for DMG weapons.

[AzazelSephiroth]

I think the amount of 50,000g sounds about right perhaps a bit less if instead of 12th level- anyone with out the ability to sneak attack took a point of damage. Or you can always use the old stand by one negative level as long as anyone without 12 level possess it.
I would note that some of the most unique weapons like this one are not usually purchased or made by PCs but found or won through adventuring. So as long you like the idea of the weapon in your game or your DM allows it, I don't think the amount is that important.