Not a parent, but my parents growing up paid me $20 when I brought home a report card of straight As. No sliding scale shit, it was all or nothing. I think that worked pretty well honestly, at least until I was old enough to understand that $20 really wasn't enough for shit, because I worked my ass off for those As. I remember the first time I brought home a B, I think in the 6th grade, I had never felt so disappointed in my life. I pleaded with my dad to cut me some slack, maybe slide me a $10 since the rest of my grades were still As, and I swear he looked me in the eye and said "No son, you know the deal, it's $20 for straight As. I hope you try harder next semester". And I'll be damned if I didn't, that was the first and last B I got until high school when I got a job and made 20 bucks for just sitting around for 3 hours.
I knew parents who did that sliding scale stuff though, which made me jealous at the time, but as an adult I don't see the point. I think there's merit for incentivising being the best you can possibly be, but if you're going to give a reward for half-assing it, you'll probably just half-ass it. My best friend's parents would pay him more than I got for straight As when he got straight Cs, so guess what, his grades were just kind of shit.