That's interesting! I find the all-cotton beds to be firmer than the latex models by quite a wide margin! That 5" topper is a big boy....
I sell that combo for $ 5,716 delivered (add sales tax if in VA), but that's pretty close to that price you found. (std line)
There is a difference in "latex", there is man-made synthetic and there is natural. Of course, the natural latex is far more expensive. When shopping beds, see if you can find out the content of the latex. Natural is better and lasts longer. On Royal-Pedic beds, their regular line latex is a blend of 70% natural/30% man-made, and their organic line is 100 % natural (and more expensive as a result).