Hi Hermati,
When you are comparing the “value” of different mattresses I would always suggest making “mattress only” to “mattress only” comparisons without adding in the cost of an adjustable bed so you are making more meaningful comparisons between mattresses without the price of the adjustable bed distorting your comparisons.
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 price 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) based on all the parts of your personal value equation that are most important to you.
There is some very general information about mattress budgets and price ranges in this article that may be helpful but these are very generic and aren’t a reliable way to assess the “value” of any specific mattress.
Having said that … there is more detailed information 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 to fine tune the mattress or the exchange/return options that are available to you).
If you are also interested in purchasing an adjustable bed then this is a much more objective purchase than a mattress and they can be purchased anywhere based on more simple “features/price” comparisons. There is more information about choosing an adjustable bed in post #3 here and the main adjustable bed topic that it links to that can help you choose an adjustable bed based on price vs features comparisons and also includes some retailers that you can use as good sources of information about the features of the adjustable beds they carry and as pricing references as well (in post #6 in the main adjustable bed topic). Of course there are many other sources as well and prices can change on a regular basis so I would also include some internet searching in your research and I would also keep in mind that online advertised prices are often price controlled so make sure you call the stores you are considering to find out their best prices rather than just looking at the prices that are listed on websites.
If you separate both purchases then you can make “mattress to mattress” comparisons and “adjustable bed to adjustable bed” comparisons that will be much more meaningful.
Phoenix