Silver and gold are not that rare, this Disranian trade center could have vending stands not barter stands, and currency could be in gold coins, which would be around 12-15 points each in the "standard" trade thread points system. Maybe have silver coins as well, but silver is so common :p. Perhaps q1 steel/silver coin ~ 0.2 points

.
BTW this is coming from discussion with Tonky, who said that in W3 coin system was quite nice for the BN trade center, since [trusted] newbies can come, sell all their forageables, and buy something nice with their iron coins. I think gold and lowq steel/silver coin system would be better and quite nice if you really want it to work.
However the real problem is that there is no need for currency since all villages are capable of producing all goods (very few specialist villages), and so all trades are really just trading hq crafted products/hq resources to "peasants". You're either a developed village, in which case you want curios (and perhaps food), or more of a newbie/raider, in which case you want hq crafted stuff to either improve your hunter-gatherer(/murderer) life, or try improve your industrial base. So all trades are exchanging the same stuff, curios/food for hq crafted stuff, so no need for currency.