We did DRF from Sans Souci. It was was a good thing to do from there - only a 15 minute ride. My personal opinion is that it's NOT worth the 3 hour ride from CSA...that's about 6 hrs on the bus for a 1 hour tour.
Been to Dunn's River from a resort in Montego Bay. It's a do-it-once kinda place and I'm glad we've been but knowing what I know I'd never make the trek from Negril, waste of a day regardless of price. I don't know what the cost would be but from a time perspective I'd never consider it.