In a couple years, the gigabyte will surely turn into terabyte quickly followed up by petabytes. Nevertheless, we have put together a Google Spreadsheet for the storage community. The permissions on the file allow for extra data to be added. The exponential best fit line has a very close approximation of the data (R^2 close to 1). You can use the equation $/GB = 10^(M * Year + B) where M and B come from the best fit line from the chart. As of this writing, the plot looks as follows:
This is being put in the hands of the community, but we are willing to take the responsibility of coordinating to have this data to be as accurate as possible. Any comments and extra data points are appreciated.