Cubisimo is correct....it IS possible to go to Nine Mile from Negril, and it IS most definetely considered an all day trip (and then some.) You really need to be a die-hard Marley fan...the one way ride from Negril to Nine Mile is 3.5 hours...sometimes a little more for the return depending on the time of afternoon or evening you pass through Montego Bay. The actual tour itself takes 45 minutes, you are free to wander the grounds on your own after that if you really wanted to go back and catch something (more photos maybe?)..if you had time, watching the full video in the snack shop/restaurant is well worth the time. If you factor in "pit stops" along the ride (Discovery Bay is a nice one, as is time in Brown's Town for food, walking the streets) - you've taken in some beautiful scenery, authentic Jamaican towns (vs tourist oriented ones), true slices of country life - but....are looking at anywhere from 7-10 hours in a vehicle compared then to the limited "out of vehicle" time for this tour.
Expense wise, you're also looking at the pros and cons then of a shuttle bus type tour with multiple riders vs a private driver.