John C. McCallum has collected a lot of data points for prices on both memory, magnetic disks, and flash, and made the beautiful graph below. It is interesting, since this type of graph is rather rare. He is also plotting the flip-side of Moore's law; CPU performance, but that's more common information.

Notice how both graphs are on logarithmic scales. It is interesting to see that the prices of (small) disk drives (in $/MB) has shown a consistent trend for the last 30 years. The very last data point shows a small increase due to the Thailand flood, but still of little significance when put into longer perspective. The DIMM and other memory prices are a bit more varied over the years, but again the decreasing price trend is very consistent, with a rather straight line going back to the very beginning, about 55 years ago.

He seems to update regularly, so be sure to check back on his page. (I hope the direct image link is not a problem).