Hi Need2PullTheTrigger,
Your question is much too broad for any meaningful answer and the premise it’s based on isn’t accurate but a topper is usually a single material and a mattress is generally a combination of materials and components that are layered together in different ways. Your question would be somewhat like asking why is a piece of furniture more expensive than some of the wood that is used to make it.
That’s entirely up to you. Durolux and Puralux are both different versions of blended Dunlop latex (see the latexco site here). Durolux is made in a mold and Puralux is made with a continuous pour method. Puralux is available in different blends but 20% natural / 80% synthetic and a 50% / 50% blend are the most common. It is usually used as a comfort material only and is usually softer. They also “feel” different and the choice between them would be based which one you prefer or on the specific application or layers you wanted to use them in (mattresses generally use a combination of firmer and softer layers). Firmer latex is denser than softer latex (with latex, density has a direct relationship with firmness … unlike polyfoam and memory foam). There is more generic information about the different types of latex in post #6 here.
Phoenix