Hi BSON,
There isn’t a “formula” that can be used to assess or “calculate” the value of a mattress because there are so many different variables and criteria involved that can affect the “value” of a mattress that can each be more or less important to some people and not to others who may have completely different criteria or definitions of “value”. The “value” of a mattress purchase is also relative to how a mattress compares to the other finalists you are considering or to other mattresses that are available to you in your area (or online if you are also considering online options). There are many reasons that a mattress that may be “good value” for one person or in one area of the country may not be good value for someone else that has very different needs and preferences or that lives in a different area.
There is more about the 3 most important parts of the “value” of a mattress purchase in post #13 here which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on suitability, durability, and all the other parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
The information on their website doesn’t change the type and quality of the materials in their mattress although it may lead to some people making a purchase decision based on some inaccurate or misleading information that they have come to “believe” based on reading the Tuft & Needle website. As you mentioned there are no lower quality materials or weak links in the Tuft & Needle mattress that would be a cause for concern in terms of the durability or useful life of the mattress relative to more average weight ranges (unlike many mattresses in the industry) and for some people it may be a better “quality/value” purchase than the other mattresses that they may be considering as a “finalist” but there may also be better quality/value choices available as well depending on whether the Tuft & Needle mattress was a better "match for their specific needs and preferences in terms of comfort, firmness, and PPP, on the relative quality and durability of the materials in the other mattresses they are considering, and on all the other parts of their personal value equation that are most important to them.
Phoenix