They're just packages that you download and install, so they could feasibly be charged for. I'm pretty sure it's impossible to hide the source code, however, so

They're just packages that you download and install, so they could feasibly be charged for. I'm pretty sure it's impossible to hide the source code, however, so a copy would emerge almost immediately if you tried to make a market for yourself. So yes, effectively all Safari 5 extensions will be free.