Price increases are caused by more than just over-hyped inflation. Discontinued items get rarer as more people quit, and more desired as more people join the game. More members means more nature runes, means cheaper nature runes, means more alching, means more total GP in the game, resulting in a general increase in prices. And with GP being easier to come by, fewer people are gathering raw materials themselves, and are instead just buying it all, also resulting in higher item prices.