Interesting analysis. I think it's overly simple, but the conclusions would be the same.

On the one hand, you have compared bribing cities with just leaving the money sat in your treasury. You should have either compared it with the alternative uses of your money (either building armies and conquering cities or building infrastructure) or used a discount rate (at a guess something like 5% or so).

That said, you haven't attached any value to the damage caused to your competitors, so I would expect the analysis to still be ultimately positive.

Given how cheap it seems, I think I will be bribing a load of Scipii African armies and cities when the civil war finally starts. I expect that to be cheaper than the building and maintenance cost I'd have to pay to form my own there.