Runescape price theory

Hi im new here. My friend told me about this site and Ive been looking thru some of the categories, but I havnt seen one topic on item prices. Inflation on an item or decrease. If i missed it please refer me to it. This topic peaks my intrest. I like buying alot of one item taking a break from the game for a month or so and then selling when i come back. Open for debate on any item increasing or decreasing.