The N900 was an experimental product and Nokia never aimed it as a mass market product. Although Nokia doesn't share sales figures I think it would be fair to say that the N9 will exceed sales of N900 by far. There are probably more N9s out there now than N900s. I say this because I can see N9s in the hands of ordinary users and even those getting their first smartphone but the owner of a N900 is almost always a knowledgeable geek or power user.

Practically all N900 developers gave their apps away for free. This is probably because there was little potential to commercialize their apps due to the small installed base. However the situation has changed as the N9 is a true mass market product. Hence N9 developers should not think they have to give away their apps for free. They are fully entitled to charge for their apps which now have commercial potential.

As many N900 apps can be ported to N9 without much difficulty this is a good incentive for developers to earn some money from their previous effort. As the app pool is still small, their app may be the only one of its kind in the store and hence likely to sell well. Most N9 developers have to buy their own device except for those lucky enough to get a free N950 so they should be entitled to recoup their investment.