Looking at Gamasutra (http://www.gamasutra.com/view/news/31045/Top_iPhone_Game_App...) it would appear that top-10 == $0.99. I'd guess this is because, as has been speculated here, there is an enormous number of people who are simply going to spend a maximum of $.99 on an iPhone game and that's that. Either you sell yours for $.99 or they'll find something else for that price. The evidence suggests to me that if you don't realize this, you'll simply be replaced in the top 10 by someone who does.
Of course, next week when a $9.99 game is in the top 10 I'll have to revise this opinion. :-)
Part of the reason is just because these are GREAT games that are all worth much more than 99c, but because of the fact that they are 99c, they're also fantastic value. Why would a user pay $2.99 for a fantastic game when they can get one for 99c?
Of course, next week when a $9.99 game is in the top 10 I'll have to revise this opinion. :-)