It's so much easier to obtain gold stock.And gold isn't as valuable to RS players anymore.Of course there's still high demand for fast gold, but items prices have dropped.The only high priced items now are rares which you need a high amount of gold to cover the street price or trade for rares of your own.
Its because gold can no longer buy as many things. Back then 100mil was considered A LOT, and gold cost around $4-5 per mil. This was also without the trade limit.
Nowadays 100mil is considered quite average, you would need around 500-700mil to be considered a lot. So its only natural the price of gold has fallen.