I don't know the relative cost of normal vs. magic, so I'm just going to comment on the practical aspects.
There are numerous problems with using stone balls as practice in the place of cast iron balls. The first is that, if your world has the technology required to make a cannon, then cannon balls are actually cheaper / easier to produce than spherical stones.
The second is that stone is much lighter than iron, by about half. This will make stone balls significantly less accurate. For an identical charge, they would have a greater muzzle velocity, but then be pushed around more by the air. They would slow down and drop earlier, too. You could use them to teach people how to use a cannon, but they'd be near worthless for target practice.
The third problem is that stones fracture. Some of this can be avoided by application of wadding, but there's nothing that will prevent the rock from fracturing on impact. Iron cannonballs can be recast, but stone ones are just broken.
Fourth, and probably most significant, is that stones that are hard enough to resist shattering from the initial explosion will probably be hard enough to scratch up the inside of the barrel. This would eventually result in the destruction of the cannon.
So, overall, it's a bad idea.