Regular damage to an attribute. Not a chance in Baator. Never Ever Ever.
Let me see, let's take a lasher with a +2 shadow touch mighty [+4] whip dagger an 18 strength (pretty easy to get at mid levels), and weapon specialization, point blank shot, precise shot, and rapid shot as a sample character.
At the lower range of mid levels that's 3 attacks at a high attack bonus for 1d6+9 points of strength damage each. Assuming all hit (not a very unrealistic assumption considering the attack bonusses ranged attackers get), he will deal 3d6+27 points of strength damage per round--3d3+12 if the foe makes all his saves. So, assuming the foe makes all his saves, that's still an average of 19 points of strength damage per round. A bull's strengthed raging half-orc barbarian with a modified strength of 26 will be dropped to 98 lb weakling status in one round and killed with one more blow. Dragons and bullettes will be too weak to lift their own body mass after one round against such a weapon even if they make all their saves. And even Titans which miss as much as one save would probably be dead in a single round.
Putting the ability on a melee weapon wouldn't be much better.
Consider that the rogue's crippling blow special ability (10th level minimum) deals a mere 1 point of strength damage in limited circumstances and you'll see how powerful this is. (If one were to consider that a ring of evasion is 25k and improved evasion is an ability the rogue could select instead of crippling blow, 50k might be a reasonable estimate for a Crippling blow item. Since this is not limited by circumstance like crippling blow, make it 75k base. And then since it's easily capable of dealing 10 times the strength damage, let's multiply by 10. (Although considering other damage bonusses are generally exponential, we might want 75,000^10gp) So, if it were allowed (and I wouldn't; such an item would wreck any campaign it got into), it should cost a minimum of 750,000 gp.
For a more reasonable ability, I'd say +4 bonus, deals 2 points of strength damage fort save DC 18 for half or +2 bonus deals one point of strength damage fort save DC 17 negates.