Big E,
What is least expensive to you? I have that .69 cannon that was mentioned. The 85 grns is for a projectile, I use about 150 for blanks and its pretty loud. Also, for it's size, it's a pretty strong shooter as far as throwin lead, only series of cannons that I know of that uses a patch for the ball.. The nice thing about it is that it's a little over 9 pounds, so it's easily moved about or transported. It's built like a tank aswell. Great value!
There are many cannons available, just depends on your price range. You can get a lot of bang for the buck if you buy barrel only and build the truck yourself.