As with anything in life quality inputs result in quality outputs. Excellent mattresses are excellent because they are made with high quality materials and expert craftsmanship.
There is a reason why cheap mattresses are cheap. That is because they are inferior products made with low quality materials and with corners cut in the manufacturing process.
In our society we have a misconception that the cheapest is always the best and anybody charging more is just lining their pockets with extra profit. This isn't the case. Premium products deliver better value because the craftsmanship is better, the raw materials used are better and therefore the product is better. You get what you pay for.
When it comes to making a mattress there are a lot of corners that can be cut. With comes to raw materials a producer could use high quality long staple cotton fiber or settle for a cheap low grade fiber. With memory foam they can use cheap imported foam like most do or go to the time, effort and expense to produce their own high standard foam as Essentia and Tempurpedic do.
Innerspring mattresses have a tendency to sag and lose their support over time. However the better the innerspring unit the longer it will last. Excellent mattresses have a high coil count, more turns, a better wire quality and are double tempered. Good mattresses have open ended coils or pocket springs, poor quality ones are Bonnell and continuous coil springs.
In construction if the cotton isn't stitched well it can move shift or clump over time. An excellent mattress like Royal Pedic uses a process called hand tufting, where craftsmen stitch the cotton fill in place so it will never move or clump. This is a labor intensive process but the result is a better, longer lasting top quality mattress.
Many mattresses these days are multi layered. The cheap and easy way to join these is with glues and adhesives. Not only is this not going to be the most uncomfortable but it introduces unnecessary toxic material. Essentia for example uses a process where the layers are moulded together individually eliminating the need for chemical glues.
Fire regulations these days means mattresses have to be fire retardant and withstand an open flame for a certain period of time. The easy way to achieve this result is to spray a mattress with toxic chemicals. It is more difficult and more expensive to come up with an organic alternative but this is exactly what producers of excellent mattresses have done. They either use wool or kevlar to achieve their fire retardant standard.
Good mattresses last a long time and provide excellent comfort and support. A premium mattress demands an extra initial outlay but it pays itself back over time by its durability and longevity.