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 raise

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.