I would say it depends on the company and their operating costs to define
what is a successful dvd for them. For example, companies like Wicked, Vivid,
Digital Playground spend more on their big budget productions. While small
outfits like Hush Hush take their web content and that same content later for
DVD sales and make a killing (based on interview I read with their founder).
Below are some sample quotes I found regarding adult dvd sales. To read the complete original article click on the hyperlink:
According to Joone, 100,000 copies of Pirates XXX were sold untill November. Many adult-productions only manage to sell around 5,000 copies.
The average adult industry tape sells only several hundred to a few thousand copies but is very cheap to produce with multiples of $100,000 being considered a really high budget.
Sales for a hit DVD were 35,000 to 50,000 two years ago. Now, they are about 20,000, Kenney says.

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