Watch the invisible hand at work!
Gamasutra is running an article from "Smith Sam," an executive at an Asian-based gold-farming company, about the state and evolution of the secondary market for MMOGs. It's an interesting read which sheds some additional light on this decidedly murky industry.
Since power-leveling services for World Of Warcraft started, competition has certainly caused the price to curve down. Also, thanks to the WoW expansion, it's now changed to Levels 1-70 from its original Levels 1-60. Therefore, Levels 1-60 was originally 350 dollars when such services started, dropping to about 129 dollars in today's market. For Levels 1-70, it started costing about 490 US dollars, and has now dropped to 250 US dollars.