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It's funny how I actually have to think about what "earn to stay" means.
Pay to post is straightforward enough, i grasp it as I read. But "earn to stay" requires a double-take. This could be a good thing (as you pointed out elsewhere).
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If we think it's better to not make it a tongue twister, we could do:
"Pay to post. Earn to stay."
Comparatively, it's boring, but it is explicit, brutally plain, and proposes a challenge.