From the upaseka precepts:
"He levies one sixth of people’s wealth as taxes."
I used to just think this was prosocial advice, but for much of Buddhism's history, it has been state supported. Paying taxes mean supporting state Buddhism. But Buddhism isn't state supported anymore and even when it was, those taxes paid for military and government bureaucracy in addition to Buddhist temples.
In Christianity, some sects have a policy of expecting 1/10 of income.
So, other than a rhetorical dodge (It depends!), do we have any modern examples of recommended dana as a percent of income, or failing that, numbers on what constitutes a fair contribution towards the support of organized religion in modern, western (US or European) countries?