Should be a formula that keeps increasing the tax liability per empty month, so eventually the owner has to dump it for zero or give it back to the government.
You want a number of empty units in your city so that if you want to move there is a place to move to. No empty units means someone does without. You don't want too many empty units of course. some is needed.
this price fixing is partially about units not even for rent which is a different problem you don't want. (but you want some units not for rent while they remodel)
