Hi Crandolph,
I think that most of the complexity is when the target is to "duplicate" a mattress rather than building to pressure relief and alignment needs and preferences. This becomes somewhat of a quandry when there is a particular mattress that is very close to what we want and it seems that an alternate feel just won't do the trick. This is one of the reasons why I personally put a premium on a local purchase when someone finds a mattress that is "just right".
Having said that ... for the great majority of people an online purchase that takes into account height, weight, sleeping positions, preferences and any local testing can lead to a very high quality and value mattress that works well for the large majority of people but it's the minority who are either outside of "averages" or who are looking for a particular feel where a local purchase can make more sense ... even at a significantly higher price
What I would normally do in a case like this it to make as objective a comparison as possible between what you are considering against the alternatives and any risk involved vs the savings.
In your case ... you could buy a standard layered or a "customized" mattress that had either 4 layers that were each 3" each or used other layerings that included about the same amount of material (12" of layered latex like the Elegance or the Beautiful) which for King Size would cost you under $2400 or less from several of the manufacturers on the site.
This would be compared to the King Healthy Back Elegance 12" latex for $3060 or the PLB Beautiful elsewhere which generally retails for @ $3800 (Which makes the Healthy Back an even more attractive option because it includes the Celsion talalay, the same amount of materials ... and is less).
So the question becomes ... Is the Elegance worth an extra $600 (or more with some of your online options) plus any taxes or delivery charges you may pay (with the regular price of the Beautiful it would be over $1300). I normally use a 20% difference (assuming similar materials) as a rough guideline for a local purchase over an online purchase (this of course is just an arbitrary number but it takes into account the unknowns involved with designing your own mattress) but a local purchase may have more or less value for any particular individual. Online purchases will often offer layer exchanges at a nominal cost which can give you the chance to sleep on the online mattress and then use the expertise of the manufacturer to make any adjustments that are needed based on your longer term experience after you've slept on it a while. In the worst case an online outlet usually offers a refund as well but these usually involve a shipping cost.
In the end, these are questions that only each individual can answer for themselves based on the "risks" of an online purchase and the likelihood they will be happy with their purchase vs the savings for mattresses with similar materials and quality and which parts of their "value equation" are the most important. Part of this is also based on how important it would be to "duplicate" your local choice or would a mattress with similar pressure relief, alignment, and that also took into account your overall preferences but felt quite different be OK. Either way your choices are far better than anything comparable that would be available to you at the majority of mainstream outlets or larger brands.
In terms of durability ... this will depend on the use of the mattress and the weight, movement, and sleeping habits of the people who use it. Latex is the most durable of all foam materials so you can expect your mattress to outlast any other type of foam. There is an additional advantage to mattresses which have loose layers and a zip cover though which is that with any material ... the layers in the top of a mattress will soften faster than the layers that are deeper. If an upper layer wears out before the others (or if your needs change) ... then it's a simple matter to simply buy a new layer of the same thickness and replace the old layer without having to replace the whole mattress. For most people ... a reasonable expectation would be for the top layer to last at least 15 years with a wide range of more or less depending on circumstances. There are people who have been happily sleeping on an old single layer latex mattress for 40 - 50 years (
see some pictures and video here) and only replaced them because the cover wore out (although these were typically a single slab of firmer latex and didn't have a softer latex comfort layer on the top).
Hope this helps
Phoenix