Can't remember how much it was in Ibiza - it was quite handy as we could hire them about 50 feet along the beach from our hotel in the Bay...
When I went sailing in malta last year, we had the use of Jet-skis every day for free as long as no one wanted to use them.
Obiously the paying customers had priority but our captain knew the guys who hired them out and had a word so we could use them whenever we wanted.
We only paid about 40 quid to cover the fuel - not bad - 4 jet-skis for over an hour a day for 10 days 8)
Sorry can't be more help about the costs in Ibiza - but regardless of the cost, definitely hire them once anyway - u won't regret it, they're great fun