Eeeh. For its price it is actually very, very weak. As a matter of fact, you are much better off buying a Heavy Bombard for 8000 gp (Stormwrack) and using the remaining money to get the eight hirelings needed to shoot it every round.

That actually also leaves you more than enough money to buy a barge or some other really large vehicle to mount your nice giant siege cannon on, carry your minions and party members around and... Well, you catch my drift. For the cost you gave that thing you can get a better weapon, flunkies and a boat.

Heck the siege weapon also saves you a feat since it doesn't need EWP.