
Valdast |
http://www.d20pfsrd.com/gamemastering/variant-rules/variant-rules-3rd-party /4-winds-fantasy-gaming/The-Loss-of-a-Body-Part/Prosthetics
It is third-party, so it might not be of greatest benefit, but I really go with my first choice and make it not cost anything. If you're not getting mechanical benefit, I wouldn't care.
(Depends on who your GM is though, talk to him/her about it to see what they say)

John Templeton |

http://paizo.com/pathfinderRPG/prd/ultimateEquipment/gear/adventuringGear.h tml#_prosthetic
Is a place to start, then toss in animate object I think but really this is flavor only but it is some good reference points. I would include the fact the limb would only work for him.

DajellyMan |

By "trying something different", do you mean making Edward Elric?
;p that automail arm... Your brothers armor body... You did it didn't you?! Alchemy's one and only unforgivable sin! You attempted human transmutation!
In response to your question, I would let it fly without any extra cost. As a GM, I would assume any gold required to pay for it would have already been paid and done with long before the adventure (if you lost a limb mid-adventure, then we would talk about gold costs)... And if anything, I would consider making you need to upkeep it, or have it possibly break or malfunction in certain cirxumstances

Alex Cunningham |

My alchemist lost an arm on a bad crit a few sessions ago. He's from Magnimar, so he consulted the Golemworks there who sent him to Kaer Maga, blah blah it's making for a great story hook for the character. I'll let you know how it goes, but it might involve taking the "Vestigial Arm" discovery and just re-skinning it.

Maerimydra |

My alchemist lost an arm on a bad crit a few sessions ago. He's from Magnimar, so he consulted the Golemworks there who sent him to Kaer Maga, blah blah it's making for a great story hook for the character. I'll let you know how it goes, but it might involve taking the "Vestigial Arm" discovery and just re-skinning it.
Burning a discovery for that seems harsh. Gold is a limited resource, but feats and discoveries are even more limited.

The Black Bard |

Ditto for the regeneration spell cost, maybe with a reduction if the materials actually function as said materials.
Assuming some sort of clockwork/mechanical, it would be vulnerable to heat/chill metal, rusting effects, magnetism, and so on. That said, you might be able to use it in situations where sticking something metal in would be better than otherwise, say a blade for chopping off hands. I would allow the Hardness 10 to apply.
So I would put it around 400 to 500gp midgame, or handwave at game start. Provides way more vulnerabilities than benefits, and could allow for interesting things down the line, like a custom follow up feat to improved unarmed strike that allowed you to have hardness 10 when full defending or such by blocking with the arm.