When the tour guide / tour operator quotes you a price for a private package, it is more often than not a set offer. You can always ask if they can discount the price a bit, but most likely they would respond by simply changing up the package to use other components, lower grade hotel room, less game drive time, etc.
A reputable tour company has already established the value of its programs and is not really inclined to start discounting for the sake of getting just one more booking. A small, lesser known company would be more amenable to discounting to build up their customer base and word of mouth. This is part of the reason why you rarely see the big name tours offering trips on discount offer websites like ZOZI, LivingSocial, etc.
You can always ask, but don't count anything huge (a couple of percent off).