Here’s an interesting post by an Indie Game Developer:
So Here’s How Many Games I Sell.
I get a lot of questions from young, aspiring Indie developers, and the most common query is: How many copies of a game does Spiderweb Software sell? It’s a really reasonable question. After all, making a game is a long and punishing process. It’s entirely fair to want to know what the parameters of success are. Alas, this information is generally kept secret. I’ve never given this question a straight answer, with real numbers.
Until now.
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( read the rest of the article on his website )