I will say this, I think that most customers would be absolutely floored by how little some products sell and how much other products sell. There is a very, very large discrepancy between products that are great sellers and those that aren't. And they can vary from month to month. Products that you might think are best sellers might be losing a bunch of money and other that seem like dogs could be really profitable.
Bottom line is that it doesn't matter how many copies a book sells. What matters is a) if customers like the book and buy it and b) the company making the book can stay in business selling whatever number of books they can sell.
I'll tell you a little story about when I used to work in magazines (not anything to do with WotC or D&D or Dragon or Dungeon). We had an issue that sold 100,000 copies and one that sold 62,000 copies. Which one was the success story? Yep, the 62,000 copies sold one. Because the one we sold 100,000 copies cost us a heck of a lot more to make because it had a giant poster in it and it had to be polybagged and there was additional shipping costs and art costs, etc., etc. So if you were just told the sales, you might jump to the wrong conclusion.
Bottom line is that only WotC really knows whether 4e is a success for them or not. And only Paizo can decide what is successful or not for us. Our two companies have different expectations and different owner's needs and different cost structures, and thus different measurements of success. How will you know if a company is successful? They will continue to make more products for a line and provide more and more support for the line. If that happens, then it is a success. Trust me, public companies don't continue to throw money into a black hole for long. Only successful lines...