Mostly nothing. Very many owners lay their own mooring and nobody takes any notice. I understand that the authorities are beginning to take some notice in a few places but I know that no charges have been levied in Pollensa, Zoller and a couple of others in the past two years.
Having just asked about the cost of one in Porto Colom, I can tell you it is about E1500 for a 12M for a year, from the Balearic (?)Portuarias (the council). To which of course you will need to add the cost of the guardian man if you don't intend to live aboard full time, but I have no idea what he charges to keep an eye on things or move boats in bad weather!