I don't think a promoted post should stay on there all day. I think it should be an hourly boost instead.
This way, different posts get promoted, the cost would be less, and most people wouldn't promote their post non-stop for 12 hours.
I think it might be a little more interesting to give an hour-promo boost.
This is an interesting idea. Is it bad that a post is promoted for the entire day (technically until first payout)? I'm not sure. Perhaps the length could be reflect in the amount? I'm just spitting out ideas now. Still not sure if it's a problem that needs solving, but great that you brought it up.