Hi Shellbell,
Welcome to the Mattress Forum! 
I’m glad you had a god experience there. As you know, Fox Mattress is a site member here, and I think highly of them and their products.
While the pricing is of course important, it is just one part of the value of a purchase. There isn’t a set formula that can determine this, as there are so many personal variables involved to make this determination, and there are almost as many variables involved in the materials and the construction of a mattress, all of which will have different levels of importance to different individuals (some people may desire more durable materials, others may put more stock in more “natural” materials, others may desire firmer edge reinforcement, etc.). The value of a mattress also compares to other models you are comparing.
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).
If you understand density, then you already know that the higher density polyfoams will be more durable that the lower density items. Finding out the information listed here and comparing the quality of the materials and components to the durability guidelines here will help you compare the quality of the materials or find weak links in a mattress that would be a cause for concern relative to the durability and useful life before making any purchase.
If you like, I’ll be happy to comment upon any specific componentry if you provide the complete information about the mattresses you were comparing.
Phoenix