We're taking the cruise line excursion on the everglades tour.
With Carnival, it's a price of about $60, which is reasonable, IMO. Entrance fee is about $30, so it's about $30/person for the transport (so $15/leg).
They hold your luggage on the bus while you tour the park, so it is convenient.
With a party of 4 a rental car wouldn't save you much money considering you need a larger car four 4 people and luggage (can you get a rental for much less than $120?), plus you'd have to shlep your luggage to a rental car location.
However, depending on your actual flight time, the advantage of a rental car is you may have more time to enjoy the day, as the cruise bus gets to the airport midday.