$100 will probably be the normal price in 5 years or less, but it is fun to find this stuff out.
5 years may or may not be too far out. In November 2007, after all, it was a cool $1000: http://www.nytimes.com/2007/11/17/us/17dna.html
AFAIC, the current $429 price was put in place in September 2009.
So it dropped >1/2 in <2 years, and from 400 to 100 is just 2 halvings, or ~4 years.
I expect that it will be technically feasible to offer <$100 SNP analysis on 23andMe's current scale (>=550k SNPs), but the determinant will be business factors - what's profit-maximizing? Do they have the resources to scale to demand at <$100? etc.
I think they will (lots of people and equipment being liquidated in the biotech sector will help them), and have made a prediction for during 2014: http://predictionbook.com/predictions/1677
Update: sometime in the past 3 or 4 weeks, 23andMe dropped its price to $259.
2011 has just arrived, and we only need one more halving or a sale to match this prediction! 5 years seems likely to have been surprisingly pessimistic.
I suspect this might interest some people here: for today only, 23andme is offering their full-package DNA testing for only 99 dollars (the normal price is $499).
23andme uses a genotyping process, which differs from a full gene-sequencing. From their website:
I don't have any experience with 23andme (though I seem to recall them having some financial difficulties), but the price was low enough for me to order a test.
An article by Steven Pinker discussing his experience getting tested can be found here. This has also been linked on Hacker News.