At least that's the answer based on one possible measure.
Peter Galuszka looks at firms going "dark" and thus stopping to provide financial information about themselves to the investment community and regulatory organizations. The stock value of these firms then goes down about 10%. Thus, the value disclosure contributes seems to be 10% of the stock price.
More information is here: http://blogs.bnet.com/ceo/?p=1658&tag=nl.e713