Just take the current price, and extrapolate. Not more than a few years for researchers. I don't know how retail will develop, since there may be regulations and other overheads.

For my friends in systems biology it's now almost cheaper to just send their microbes in for sequencing than doing their own gel electrophoresis.

(If you want to read around in Wikipedia, also have a look at Southern blots and Western blots, and DNA microarrays.)