I've been up and down south coast road to KGN and back many times last winter. Most of the time I used public buses that leave from Sav. If i remember good the fair was about 700 JMD. The only problem with this buses is that it leaves only when it's full, so you can never know what time you finally reach Town. This buses also get stopped and searched by police time to time, it can be quite an adventure.
There's alot of interesting places in Kingston that are off the beaten track. Make sure you visit National Gallery and The National Heroes Park. I also loved to shop for shoes at Coronation Market, and the ice cream at Devon House is just fabulouse. Anyway it's better to have a place to stay in Town, so you can spend a day, relax then go out at night and then return to Negril on the next day. But if you are planning to get a driver, that would increase the price of the trip dramatically.