This is kind of a hard nut to crack, but you can find the basic information
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Basically there is a fixed patreon fee of 5%, another payout fee of 1% (capped at 20$), a fee for converting USD to native currency (which depends on the country of residence but can go as high as 4% above the base exchange rate), and a payment processing fee which is another 3% + 0.30$ (but the way Patreon works this is shared among all the creators a patron is supporting, so for a person donating 1$ to only one creator this means Paypal is keeping only with this fee more than 30% of the payment but for people donating to many projects the fee is much more negligible as the payment is only processed once).
Long story short: the fees are no less than let's say 10% but can go much higher than that.
And all that is before taxes, of course, which are wildly dependant on the country of residence but for example in Europe can suppose quite a big bite. So when people say X creator is making 7K/monthly or 20K or whatever, only looking at the front page of the project they kind of are hugely wrong if talking about net income that is the important thing. Well, that's the cost of doing business using these platforms (that have another adventages, of course), lol.