Very impressed you made a GBA game! It's really fun, I got pretty addicted, nearly broke my finger off from tapping the button. I managed to get $10650 as my high score, took me quite awhile but I was obsessed with getting a near perfect run. I didn't manage to buy at $5 and sell at 85 though, it was 7/82, and I imagine with buying and selling a couple times in a run maybe you could do even better if the gods favor you.
My only wish is being able to hold the button to buy.