Quite simply, if a page is in the root(and/or subdomain?) then it's considered top priority, if it's in a folder below the root then it's next to top priority, if it's in a second folder then it's the next priority and so on. So the further from your root folder you get, the lower the priority is. With perhaps the index page being assigned priority over other pages. So...
example.com = Priority 1.0
example.com/test = Priority 0.8
example.com/test/test2 = Priority 0.6
test.example.com = Priority 0.9
...and so on.
One also might want to see greater priority given to pages without a query string, with priority decreasing as query parameters are added. So...
example.com/index.php = Priority 1.0
example.com/index.php?do=test = Priority 0.9
example.com/index.php?dothis=test&dothat=test = Priority = 0.8
...and so on.
The numbers were just demonstrative, no doubt there's room for tweaking. But I think it would be useful and easily within your admirable expertise to add a little artifical cleverness into the algorithm so it can take a best guess at what priorities go where.
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