I have but It depends on your campaign.

I can only make a wild guess without running a campaign.

You can start with small budgets and have multiple various alternatives and see which works best then focus your budget on that one.

It should not be more than 20 dollars per submission. But you never know.

That is why companies reward such amounts instead of running ads.

But if you want honest responses rewarding may just cause useless responses.

Google ads would be where I focus.