I am a bit confused, you booked flights and car rental now but when are you actually going to SXM? Prices sound like they may be ok for High season but not for Low season.
I may be wrong but I think CDW is Collision Damage Waiver, which you sign so that your credit card company (check to be sure) will cover any damage to your car and should cost you 0. I have seen Liability coverage range from $5 to $12/day, dependent upon car rental company and $ coverage.