Drf is in Ocho Rios and not worth the ride from Negril. You could do Mayfield falls but it would be an extra cost. I haven't done it but have read that it's beautiful. Wait until you try one of the Ochi resorts before trying drf.
I agree with softail19, but to answer your questions:
The entrance fee is $20/adult. It is about three hours from CN, so it would be a full day excursion. There are tours from Negril that include transport and entry fees. Check the schedule to see what cruise ships are in port on what days and avoid the days with very many (typically, Wednesday to Saturday), not just in Ocho Rios but Falmouth and Montego Bay too. Some Wednesday and Saturday's there will be five or six ships with over 12,000 passengers, half of whom seem to want to go to DRF.
DRF is a free inclusion for both CSS and CTI, but not CN and CSA.
At least 3 hours from Negril. If I were in Negril I would lean toward Mayfield Falls or Y/S Falls as they are closer. They are not as developed at DRF, but to me that is a good thing. At DRF when you get out of the river you sort of wander through a village of vendors who are eager for your business, some a little too aggressively for my taste. It is a must-do once though!