As Tres says supply and demand.
I think a couple of companies have scaled down production/pulled out of the consumer ram market in the last while, as well as thngs like moving fabrication plants over from RAM to Flash memory (which I think was more profitable for some companies).
Also more ram is being used in mobile devices (decreasing stock available to be put onto ram modules for PC's), and a new major chip has just come out which will likely increase the number of prebuilt systems sold for a bit.
I remember seeing ram literally doubling in price in the past after things like earthquakes, as well as plummeting through the floor (comparatively speaking) when a company has decided to pull out of the market/needed the cash urgently and dumped stock from it's warehouse.
At the moment you can get 16GB of decent DDR3 ram for around a hundred, about 4 years ago that was 4igb of DDR2, a few years before that, 1gb of DDR.
Unfortunately you can never really predict ram prices, as there are so many factors that can influence them, everything from the weather in Asia (extremely heavy rain can lead to a fabrication plant losing power and having to shut down*), to the exchange rate between the UK and the US
One thing you can normally predict is that within a couple of weeks of me buying ram for my home machine, the price will half.
So I'm half expecting 32GB of ram to drop to £100 in about a week as I bought some last week
*Or in the case of the HDD manufacturers, flood several of the major factories, and worse than that, flood a number of the big suppliers of parts for the drives (thus affecting drives being made in factories in other regions).