One alternative would be seizing unused addresses (as they are supposed to be) and auctioning them off. Repeating every so often if they're still unused.
This would raise prices (in the event that is really desired) while avoiding squatting and rent seeking.
Unused addresses often aren't. You might not be able to connect to them, but that doesn't mean disuse. There are various networks around whose operators want them to have unique addresses but not be connected to the global internet.
For example a power company I know about. The operators there decided to use public addresses for the power control/monitoring network instead of 10/8 to be sure of having unique addresses, even in case of a merger or cooperation with another power generator.
This would raise prices (in the event that is really desired) while avoiding squatting and rent seeking.