Does the base tax of colonial nations contribute anything to the base tax comparison when trying to vassalize a nation? I have a bunch of wealthy CNs, Caribbean etc., but even relatively small nations (e.g., Genoa, total base tax 10) have a modifier like "Portuguese Base Tax compared to Genoese: -50."
It most likely doesn't. Only vassals BT in incorporated into your tax base, so I guess only vassals will give you profit here.