The monopolies should act, well, monopolistic.
And the main difference is pretty easy to explain, and the explanation works well within the economic model of V3 (where prices are a direct function of supply and demand).
A firm on a competitive market has no chance of changing the prices by manipulating output levels.
So when it wants
to maximise profits with constant production costs, it has to
maximise its outputs.
This behaviour is beneficial to the society as a whole, so the goals of the individual profit-seeking agents and the society at large (aka player) are aligned.
With monopoly, it's not the same.
A monopoly has
1) cash reserves and market power to buy out or suppress those who try denying it the next point
2) options to meaningfully inflate market prices by
reducing output
That means that the effective level of production (the one that maximises its profits) for a monopoly is lower than the maximum it's able to get, and in that, its interests are not aligned with interests of the player.
That's what's harmful about them, and that's what I'd like to see implemented in game: monopolies actively trying to corner the market (by having the mechanical ability and desire to buy out independently-owned industries in their category) to jack up prices by shrinking the supply.
If it's unclear why reducing output can be beneficial for a monopoly, here's a spreadsheet:
| Monopoly's output | Non-monopolistic supply of the goods | Total supply of the output goods | Total demand of the output goods | Price of the goods | Input and labour cost per unit of goods | Profit per unit of goods | Total profit for the monopoly |
Scenario A (maximise output) | 1000 | 200 | 1200 | 1200 | 20 (base price) | 18 | 2 | 2000 |
Scenario B (maximise profits) | 400 | 200 | 600 | 1200 | 35 (+75%) | 18 | 17 | 6800 |