Ah, I got it. If I'd engaged my brain for a second I should have gotten that. I presume I can use the example and add other breakpoints like
(abs([Quantity] >= 5 && [Quantity] < 10)?[Price] * 0.05:0) + (abs([Quantity] >= 10 && [Quantity] < 20)?[Price] * 0.1:0) + (abs([Quantity] >= 20 && [Quantity] < 30)?[Price] * 0.2:0)