Apropros of this post, here are the options for Neck armor in my DF game:
Greathelm: includes coverage for the Neck. Excellent choice at DR 7.
Mail Coif: includes coverage for the Neck. Acceptable choice at DR 4(2), but it also doesn't provide a problem with vision restrictions.
Anti-Garrote Collar: +4 DR for the neck vs. strangling, not blows. Not sure exactly how that works (I assume it's leather stiffened with metal wiring, so it doesn't collapse easily but doesn't help much against impact.)
And that's about it. Otherwise, your neck is wide open to attack. It's -5 to hit, though, and it's not common for monsters to go for it for that reason (a -5 off your attack skill is giving up a Deceptive Attack -2 plus change.) It's a great target for skilled fighters vs. moderately armored foes, or patchwork armored foes. It's not a bad choice for arrows, either, because it's rarely got more than 4 DR . . . still, most Heroic Archers shoot the vitals with armor-piercing arrows for the x3 multiplier.
Any other options? I should probably allow a couple, just because this post will instantly make my players paranoid. How about these?
Leather Neck Guards: An aventail/gorget combination of stiff leather. DR 2. $10, 0 lbs. Commonly attached to leather helm.
Scale Neck Guards: A lobersterback/gorget combination of scales backed by stiff leather. DR 4. $50, 1 lb. Commonly attached to a barrel helm, pot-helm, or legionary helmet.
Plate Neck Guards: A lobsterback/gorget combination of segmented plate. DR 6. $125, 2 lbs.
The above neck protection can be layered with other neck armor; this gives No Peripheral Vision (you can't swivel your head freely!), or Tunnel Vision if you already have No Peripheral Vision.
In almost all cases, you're probably better off with a Mail Coif (or a Greathelm, if you are adding to a Barrel Helm.) Why is that? Mail is DR 4(2), $55 and 4 lbs. and also covers the skull. You can layer a lot on the skull - IIRC, Vryce has a Mail Coif under a Pot-Helm and puts on a Greathelm when fights come, for a base non-magical DR of 17(15) including Skull DR. You can even put a Mail Coif under a Leather Helm and get solid skull, face, and neck protection.
How did you calculate them? Basically it's 50% of the cost of a helm that provides the same protection, and adds 20% to the weight, rounded off.
How about Face, while we are here? Yes, you can add Face Guards to a Pot Helm. It's $50, 1 lbs. AKA, just take a Legionary Helmet.
So my torso armor doesn't protect my neck? No, not since 4e. 3e GURPS had it use the greater of Head or Torso armor, per Compendium II p. 52. 4e changed that.