I'm planning to develop an open source application and am also considering to set a non-zero price for it.
If an user purchases the app, will he be charged again when updates are released?
From my experience with the Software Center and the Humble Indie Bundle apps, no, they will not be charged again. I've installed updates to Bastion without any re-payment, though I redeemed the Bundle apps via the gift codes, so it might be different for store-bought apps.
When you purchase an app, it adds a private repository (a.k.a. PPA, or personal package archive) to the folder
The answer is no, they will not be charged again if updates to the app are released.