I don’t know what it costs, but adding an axle doesn’t come for free.
If it did, those axles would always be on the ground. So-called “lift axles” can be raised because keeping multiple sets of wheels on the ground makes cornering harder, increasing tyre wear and energy usage.
If it did, those axles would always be on the ground. So-called “lift axles” can be raised because keeping multiple sets of wheels on the ground makes cornering harder, increasing tyre wear and energy usage.