The prices of gold can be looked at in several ways. First of all, it is quite obvious that the value of a Gold Piece in Runescape has greatly changed over the past few years. Back in summer of 2005, the most expensive Phat, a Blue, was roughly 50-55M. This is a difference of nearly 1000% between now and then. Other Rares reflect similar changes in value through the same time period. Back in 2005, the average player had much less GP, because of the basic mechanics in Runescape, which fueled the inflation of GP.
Inflation has been going on in Runescape forever. The x factor was the Dupe, which drove prices right back down to near their starting points. However, the patterns continued after the Dupe, up until around summer of 2006. At this point, Jagex introduced many new facets of the game, such as Construction (natural money-drain), and Skill Capes (complex money-drain). Since these implementations, the Rares market has seemed extremely static. Prices have actually slowly dropped on most Rares, Phats being the exception.
It is only natural that Real-Life prices reflect the In-Game prices. The reason GP was worth more in Real Cash back in 2005, was because GP was also worth more in Runescape. If you showed a player 50M back in 2005, they'd say you're rich. If you showed them 50M today, they'd say "who cares". Obviously, there are other serious factors, such as Gold-Farming and Autoing, but these often are huge fuellers of inflation anyways, so they go hand-in-hand.