Balancing time.
Comparing to Dagger and Short Bow.
Dagger: 2 (attack) + 1 (booster ability) - 3 (permanent slot) = 0
Short Bow: 3 (attack) + 1 (booster ability) - 3 (permanent) + 1 (ranged) = 2
This: 9 (attack) - either 2 or 3 (negative ability) - 3 (permanent slot) = 4 or 5, adding, say, 2 because it's in Other.
I would definitely suggest it be higher than 2 so that it does not greatly eclipse Hammer
(I would also contest Ranged counting as +1 since airborne does not and web is situational)
Short Sword: 3 (attack) -3 (Weapon) = 0 elemental = 1 other = 1
Hammer: 4 (attack) - 3 (Weapon) = 1 elemental = 2.5 other = 2
[Equation used: 1+1.5E=O]
Halberd: 9 (attack) -3 (Weapon) -C (2 handed) = 6-C elemental = 10-1.5C other = 8.5
of C=1
Personally I think that Short Sword [3], Hammer [4] --> Halberd [9] is a huge exaggeration. Short Sword [3], Hammer [4] --> Halberd [6] would fit my experience from D&D and other RPGs much better.
Halberd [6|9] would cost
Halberd: 6 (attack) -3 (Weapon) -C (2 handed) = 3-C elemental = 5.5-1.5C other = 4
of C=1