$100 is a cost of a 60mm mortar shell. It is a hand grenade sized munition lobbed from a Pringles can sized weapon to the range of 1-2km. This is generally not the thing that comes to mind when you think of artillery strike.
A 155mm (dumb, unguided) shell would set you back 5-8K USD. That's before the propellant charge, fuse and amortization of the artillery piece and its 5 man crew.