Pricing proficiency

I would like to price creation of magic item granting proficiency(like… sword of berserkergang), but I cannot find a way to do it. How should I price it?

IANA, but 1 custom power, which can be one proficiency, is equivalent to a 1st level spell used once per hour, and a permanent spell effect usable once per hour is 500gp x spell level x 16, so that sword of berzerkergang should be +8000gp.