Isn't $3.99 on the high end of app pricing? Instapaper sells for $4.99 and is considered a premium product.
How long does it take to make a $3.99 app? Do you factor in those development costs?
If it's just one person and you make $50K, that's fine. But what if there's two of you?
What's the probability of getting 50 sales/day and for how long? In the event that you get, let's say 25 sales/day, what's your fallback strategy?
awolf, you may very well be selling hundreds of copies a day of your apps at 10 bucks a piece, but the point is that the vast majority don't.