Because of the inlfation on the runescape market, all of the botters, pvp worlds, and the easy money making methods the gold prices became so low. I doubt they will rice again. Or Someone have to remove 1000's of billions rs money from rs.
It's because of all these bots. Back in 2007, bots would only gross about 100k an hour. Nowadays they're making 400k+ an hour and the bots themselves have improved greatly.
Prices will never go back to $4/m. I'm hoping they'll stay around the current price, but it's unlikely due to the ease of getting GP.
One of the few ways that the price of a mil could increase in value is if Jagex introduces a huge money dump skill (construction V2, Summoning V2, etc) which would remove massive amounts of gp from the game. They would also have to start banning bots massively (good luck to them )
Even with a new money dump skill where little to no returns are made all that would happen is the demand for gold would increase, but then supply/ # of bots would also increase.
Econ 101 ftw.