If we need mint imperials for catapult ammo then I can get 3kg trade for less than £10! Aniseed balls would be a little more. The cheapest would be 3kg of jelly beans at less than £7 but then they just don't seem to have the penetration or aerodynamics!!!
If we use standard bean cans for targets then that should present enough of a challenge. Points could be awarded for either speed or the number of shots it takes to clear say five cans.
We could have a catapult field course like they do for Field Archery. It could consist of different distances, targets and obstacles to shoot around, under, over or from. I can even borrow steel mini popper targets that are hinged if you want to get really flash. They are about the size of a one litre bottle. I could even set up a catapult night shoot.
I would be willing to run and score it. I guarantee it would be really fun and I'd provide a prize.
Phil