The drop rates are unfortunately unknown.

If you refer to lucky star wheels, the drop rate of jackpot is likely to be very very low. I personally spent somewhere between 200-300k cps on that event over the years and never got a jackpot, and I'm not alone with this. Don't do that event for the jackpot, do it for the other stuff, the jackpot is just a nice addition if it drops. (Goes to pretty much almost all other events as well)