How much would a 14K amethyst and diamond yellow gold ring cost at average?

It depends on the carat weights of the stones and the weight of the gold in the ring. Amethyst is one of the least expensive gemstones (I'm assuming your looking for purple amethyst, there is also green amethyst). The cost of the ring will be determined by the carat weight and quality of the diamonds and the gold weight, and most importantly where you buy it. I tell people to never buy jewelry at malls, the quality is very poor and the prices are marked up tremendously. I would look for an online jeweler who has reputable feedback. Your more likely to get a better price because online jewelers don't have the overhead costs of a jeweler with a physical location. I wouldn't pay more than $500 for the type of ring you are looking for unless it's huge with large diamonds. A nice size amethyst stone with diamond accents in 14K yellow gold will most likely run you 150-400 depending.