According to Wikipedia, Consumer Lab, an independent dietary
supplement and over the counter products evaluation organization,
published a report on 13 November 2007 on the popular resveratrol
supplements. The organization reported that there exists a wide range
in quality, dose, and price among the 13 resveratrol products
evaluated. The actual amount of resveratrol contained in the
different brands range from 2.2mg for Revatrol, which claimed to have
400mg of "Red Wine Grape Complex", to 500mg for Biotivia Transmax,
which is consistent with the amount claimed on the product's label.
Prices per 100mg of resveratrol ranged from less than $.30 for
products made by Biotivia, Jarrow, and Country life, to a high of
$45.27 for the Revatrol brand. None of the products tested were found
to have significant levels of heavy metals or other contaminants.