Unless you absolutely have to do a particular excursion it's ok to wait and do it on the ship. They can sell out but it's not likely unless you wait until the day before your in port at 8pm and then want 4 seats.
I usually plan $100/day for expenses for each port day including excursions, so a 7 day trip with 3 ports, $300pp extra. If you simply plan on a beach trip, wander around, go by taxi $50pp/day is probably fine. Plan more if you drink a lot though.
Most agents will include an on board credit or a better price than through the cruiseline. Always get something even if it's a bottle of wine or free coke plan.
5 day trip should be about 2-300pp for inside cabin. Sometimes you can get additional people in the same room for much cheaper, say person 3 and 4 for 100 each though I would recommend 2 rooms...they are small.
100pp per port day(assume 3 ports)=1200
=3050 minimum, with 5k you will be fine to drink or gamble a bit, do some better excursions like the dolphin swim, some shopping etc.
Cruise experience: 15 cruises, last one was 30days in May