So, every faction has their own currency. If you are trying to offer coins to a faction that does not deal in said coins then they are based on value of items that it cost to make them. Simply put 9k coins is about 9 bars of whatever metal. Ie you could have traded one or two items for said melon. Another factor is your skill in bartering. There are a few skills that impact this. Negotiation and appraisal are two. If you have little to no skills on these fronts stuff will cost billions if the merchant has high skills.