Aurora takes nothing out of a donation, on every plan you can sign up for. The ledger beside this has no line for us because there is no line to draw — the one deduction is the processor's, and the arithmetic comes out the same whichever plan you are on.
What Aurora costs sits outside the gift entirely. On Free you are billed nothing at all and your donors are asked to add an optional contribution at checkout, which they can decline. Pay per update charges for the writing instead: an update — one thing you have to say, turned into a post for every network you are on — costs $0.99 once you are past the 2 a month that are free, and a month's worth is billed together at the end of it. And Starter at $29/month, or Growth at $99/month, is one bill and no counting.
Aurora takes 0% of PayPal donations — PayPal's own rates apply. On a PayPal gift that zero is structural rather than a price we picked: there is no mechanism in Aurora for taking a share of one, and no setting that turns one on. PayPal giving is included on every plan, including the free one. Aurora takes no share of a PayPal donation at all — it settles straight into the nonprofit's own PayPal account.