Hi hickoryboy,
[quote]Question - like everyone, I’m looking for the best value in a mattress - best mattress/material/durability/comfort for my $$$/budget. Would getting a less expensive/durable mattress (innerspring, polyfoam) and putting a high quality latex topper (3" or so, mid 20’d ILD) be a viable solution for saving $? Since I would only have 3" of latex and 4" or more of less expensive material. Would the highly durable 3" of latex “protect” the less durable material from degradation and allow it to last longer?
Or is this even a good question?[/quote]
Alll questions are good ones 
Your question would depend entirely on the quality, thickness, and firmness of the layers that were on top of the mattress you wanted to use as a base. Most mattresses already have several inches (or more) at least of softer foam on top and if this is made up of lower quality materials then the layers that are designed to provide pressure relief comfort end up being moved to a lower position in your sleeping system and become part of the transition or support layers and in general they are too soft and not durable enough for this. You would be using comfort layers as support layers in other words and this is very risky for alignment.
While this can be a good option if someone has no other choices available (such as a mattress exchange where they need to choose from the mattresses that the store has available) … because most mattresses include comfort layers it can be a risky approach to use if you don’t have to.
So if you can find a mattress that has no more than two inches of polyfoam that was firm enough to be used as a transition or support layer and was good quality then you could add a topper to this that was a suitable thickness and softness for your body type and sleeping positions. As an alternative you could use a version of this same mattress that used the same thickness comfort layer but in a softer version and then add a thinner topper to “complete” your comfort layer but this would still mean that part of your comfort layer (the weak link of a mattress) was still lower quality foam and while it may be “more durable” than it was without the topper … it would still be less durable than if the entire comfort layer used higher quality materials. In most cases it’s better and more cost effective to buy a mattress that already has the design you want (a less expensive base material and a high quality comfort layer) than it is to buy a mattress with the intent of adding a topper as a comfort layer over pre-existing lower quality and/or softer materials.
For example … there are many mattresses that use firm good quality polyfoam support layers with about 3" of latex on top that are available for well under $1000 (queen) which don’t have the “weak link” of several inches of soft lower quality polyfoam in the mix. A good innerspring/latex hybrid may be a bit more. If you add a couple of hundred more they become even easier to find.
If you could find a mattress that only had 4-6" or so of good quality materials with no low quality foams in the mix and they were firm enough to be used as a support layer and the only problem was that they were too firm to sleep on directly, then this would be a viable option but mattresses without a comfort layer at all or that use better quality firm materials in a thinner comfort layer are very difficult to find inside a reasonable budget that still allows for the purchase of a high quality latex topper.
A latex topper would allow the materials below it to last longer yes so they would be more durable but the overall sleeping system you end up with may still not be suitable to provide you with good alignment or may still have a weak link in the comfort layer.
Manufacturers will often put higher quality materials over lower quality materials deeper in the mattress to “balance out” the durability of the layers as a way to lower the prices. You will often see for example premium foams (such as good quality latex or memory foam) used over a couple of inches of lower quality polyfoam (say 1.5 lb density or even lower) but these have a firmness level that is chosen as a transition or support layer not as a comfort layer and there is often still “issues” with this type of construction depending on the body type of the person and how deep inside the mattress the lower quality materials are.
The icomfort Genius for example uses 2.75" of their gel foam over a couple of inches of “support foam” and then this is all on top of the base polyfoam layer. This “support foam” that is used in the comfort layer is only 1.35 ls density and many retailers have commented to me that this layer in the Genius has softened significantly in their floor models so when a customer gets a new mattress it is nothing like and much firmer than the floor model they tested.
Phoenix