I think the efficiency of aerial bombs vs. cannon fire depends on several factors to quantify this. I don't think it's that easy to make this comparison. Especially because cannon fire does not replace aerial bombs.That is cheaper than cannon rounds. Before the extensive use of FABs, the Russians routinely fire thousands of rounds of cannons and kill and wound Ukrainians by the hundreds per day. If you do the math, that was about 20 rounds of cannons fired per each Ukrainian kill or wounded. Given that a round of cannon shell cost about $500, that is $10000 per Ukrainian killed or wounded. Each FAB kill or wound at least a dozen people in a building, that comes to $1000 per Ukrainian killed or wounded.
Furthermore, Russian cannon fire has improved greatly in terms of accuracy, with the aid of drones correcting artillery fire and better data fusion systems (ARM-K-M - the Russian copy of the Ukrainian Kropyva), the Russians have become They have improved a lot compared to the period when they entered this war in February 2022, but the Russians are certainly still behind the Ukrainians in this regard, due to Kropyva command and control systems, among others.