There is now an organisation that manages many of the sites in Menorca, with an office in Mahon – Mao. As a result many of the popular sites now ask you to pay a few euros.
I visited in July 2013 and on Mondays access to the sites are free, so you might want to bear this in mind.
If you visit here, be sure to continue for about 750 metres down the lane to Sa Comerma de sa Garita and Ses Roques Llises. I put a map on the page for Na Comerma, which is more commonly called Sa Comerma.