following Jack’s suggestion , I had a look at the Open Budget Index.
Very interesting: they developed a system to measure the transparency of the budget in 59 countries.
Based on a questionnaire survey, and on analysis of the policy documents, they arrive to the following index ranking (only top10):

  1. France
  2. United Kingdom
  3. New Zealand
  4. South Africa
  5. Slovenia
  6. US
  7. Peru
  8. Sweden
  9. Poland
  10. Brazil

Interesting mix!

One of the interesting things they do concerns the classification of “stages” of availability of documents:
A. Not produced, even for internal purposes
B. Produced for internal purposes, but not available to the public
C. Produced and available to the public, but only on request
D. Produced and distributed to the public (for example, in libraries, posted on the Internet.)

My main criticisms about this methodology: it’s expensive as it involves questionnaire surveys (rather than website survey); the index is too complex, a compound indicator is always more disputable.