Yeah, I know what you mean. I'm still on an upward trajectory and earned $425 in December. I published 36 stories, which is about $11 per story.
But about 30% of the biggest earners were stories I'd published in previous months. So it's not a straightforward cause and effect, but it's probably quite representative of average earnings per piece.
I do enjoy it though. Am a bit addicted, but need to reduce time on here in the new year. Perhaps not try to force it if I have nothing to write, but just focus on some possible big wins. And go back to re-publishing some older material. Some stories are predictable to do quite well, but some that I think will do well, don't.