I too have thought about how ads in paid apps seem "rude," but lately, I have begun to wonder if there is actually a middle ground. If you decide to sell your application for a low price when you know that users would be willing to pay much more, is there something wrong with using ads if you let the user know before they purchase? I am pretty conservative when it comes to buying mobile apps, and I think I'd be a lot more likely to pay for a $0.99 app with unintrusive ads, than pay $5.99 and never see ads.