It depends on the yacht you pick, and how many persons are in your group. Small basic private yachts for 5-8 persons start from approximately $600 for 4-hours, and the larger and more luxury ones for 20-30 persons will be $2000-$3000. Also most yachts more expensive on a weekend/public holiday than on a weekday.
Depending on the yacht, you can bring your own food and drinks onboard for no extra charge, but usually if you want to rent a BBQ grill on board you'll need to pay an additional charge.