How does it work? I'd love to hear the mechanics. A boud elemental? a tiny portal to the plane of fire? Do you reload it with small boulders, or does tha ammo just form inside it?

Personally? I'd make it a medium two-handed ranged weapon that requires X strength to be used properly. Definitely reduce the price to something approaching normal, the investment of EWP seems like plenty for me. Have it reloadable with small-sized boulders (akin to giant rock-throwing boulders) as a full-round action, either on the shooter's part on the part of an adjacent assistant. So it's sorta halfway between personal weapon and siege weapon.