I'm not fluent with code, but combining what I see in the code with my experiences challenging the arena, this translates to:
defeat rank 1 arena deck: 100% bonus
defeat rank 2 arena deck: 50% bonus
defeat rank 3 arena deck: 25% bonus
defeat rank 4 arena deck: 17% bonus
defeat rank 6 arena deck: 8% bonus
defeat rank 7 arena deck: 3% bonus
Edit: rank 7 definitely equals 3%. I don't understand the maths, but that's what came up when I battled just now.