Are you sure about the 15$ thing? I mean, honestly this game is not worth 15$, but I think it's worth 5$...
If people don't think it's worth $20, they generally don't think it's worth paying for at all. Because at $5, you still have to fill in three pages of forms, you still have to wonder if you'll get anything, if the site is secure, etc. You still have to go and dig the credit card out. So it's only worth $5, why bother?
And yes, there's a lot of evidence that lowering the price below $20 has no positive effect ( even sometimes negative ) to sales.