firstly, I believe its confirmed that an established currency will be in-game at launch, and player-created currencies was talked about by SBS at one point...a Q&A but I cant remember which one.
second, I'd imagine NPCs will have intrinsic values at which they'll decide if a coin is worth the work or not...but AI has always been stupid, and I wouldn't get my hopes up if I were you. If anything though, they wouldn't compare it to one particular item or thing. Probably would do a quick check against the usual rate for the specific work involved, the amount accepted by other NPCs and/or players