do the apps that are sold in MAS have free updates?
if not , how can I found out what the developer has decided for his product ?
so far , for the apps that I have purchased there have been some updates for free .
Yes. Updates are free.
You can check the developer site for update information.
Launch an app then from the menu bar (top of your screen) click the app name then you should see Check for Updates from the drop down menu. That should take you to the developer's site. Or look at the app again on the MAS. You should see a link for developer support.