It is basic supply and demand. As the population levels up the higher end mats and the lower end mats will drop in price (higher lvl pop can farm more high mats and they also tend to alt which means lower end mats get common again). I would gamble the items which have seen stable prices or slightly higher prices are items found between 35-60.
I expect the most items on the two extremes to drop in price for the next week (weekends when the biggest drop happen) while at the same time seeing a slight uptick in price of mid grade items.