You two are not giving up, are you? Okay. Turkish raids probably did have some effect on taxes. But, as far I know, serbian rulers took 1/5 of everything while ottomans took 1/10. Again, as far as I know, 1/5 was the standard practice for all serbian feudal lords, even before ottomans came to balkans. That only goes for the first period of ottoman rule.

@Duke
Main source of revenue doesn't have anything to do with how high are the taxes. Mining was very developed in serbia at that time, I agree, but that doesn't automaticaly mean the taxes were low, does it? Dubrovnik paid special tax, I think it even had a name, (I am not sure, it's been a while since I read serbian history books), to remain nominally independent.