This.OP didn't talk about Multi-Cannon. he talked about Cannon.
As I wrote later in this thread - MULTIcannons are on the slightly expensive side, but still viable. If I needed around 900 Cr to kill a (very passive, so I barely missed anything) Cobra, then I find it more or less balanced.
On the other hand CANNONS are not balanced at all. I can't say how much the ammo to kill the same Cobra would have cost, but it's certainly at least around 5k as cannons are way less accurate than multicannons, so you can't assume you can hit with every shot.
So, even to hunt the "really big fish" (with bounties of 100k) it's not viable. Because bigger ships need also much more ammo expenditure, meaning that it's not realistic to kill a 100k - bounty Annaconda expending 10k of ammo. Probably around 30k at least. So you would have a loadout that would make any profit ONLY if you fight high bounty
targets and never fire a shot on anything else. Considering how many high value targets are flying around (hint - not so many...), it's definitely NOT a viable option. Especially as you can have the same (or because of better accuracy - rather higher) firepower if you install multis.