How much would you pay for a Helm of Protection from Evil?

[Empirate]

Helms and other Wondrous Items don't have a spell failure chance (for arcane casters is what you mean, I presume). Only actual armor and shields have that, and helms aren't technically part of that category. In fact, heavier armor usually comes with a helmet included, but that helmet can be exchanged for a magical helmet without impacting the armor's statistics.

 

[jefgorbach]

Where do you get the 10 HD thing from?

Detect Evil.

Presumably the Protection would only be effective for the 6-36 seconds (1d6 rounds) following an evil act during which creatures with 10 or fewer hit dice are Detectable as being evil.

 

[Empirate]

Protection from Evil doesn't use a Detect Evil effect to function. It simply checks for alignment: Is the thing that's trying to hit me evil? Yes, enjoy your bonus, citizen!