Its 2 euros one way, 3.75 if you buy a return ticket as well (be aware though - we bought return ticket and he ripped them - on our return where he claimed our tickets were used because they were ripped)
It's a small boat but it was never too crowded (except once when we took the night boat to El Divino) and it's a comfy ride (despite its size) - we were never bothered by waves nor did we get wet.
At night the boat shuffles between Ibiza town and El Divino (1 AM - 4 AM).

