Many consumers now shop online due to convenience and lower prices. Brick and mortar stores must compensate for rent and salespeople, so prices are often higher.
The answer really depends on you and the mattress' intended implementation. Let's look at sizes (manufacturers may vary slightly):
There are three parts to consider regarding a mattress purchase: padding or upholstery, inner springs, and a box spring or foundation. People often associate the word mattress with the top layer of the bed — the padding. The mattress padding can consist of three-to-eight layers of padding. The layers consist of man-made materials, natural fibers, and a variety of foams. Padding quantity does not always equal quality — that is why it is important to test certain inner-lying materials. Materials such as wool, silk, and memory foam / latex foam are commonly used in mattress padding. Established brand names and quality of certain padding varies. For instance, foam padding in a $2,000 model will vary from the foam padding in a $3,000 model.
Innerspring coils allow for even weight distribution while sleeping. A mattress' price is usually contingent on the number of coils used in its makeup; yet, like padding, quality of coils differs, so more coils do not necessarily mean a better quality of bed or rest. Coil design, function, and durability vary amongst manufacturers.
Box springs are affixed with mini coils, which help supplement weight absorption. It is believed that a quality box spring can help extend the life of a mattress set. Some mattress sets come affixed with a foundation rather than a box spring. Foundations can be framed in wood or steel, yet do not address shock absorption like box springs.
It is recommended to give some thought to the purchase of a water bed. Waterbeds may not be for everyone, yet waterbeds do offer some advantages over traditional beds. Water beds can be heated; the life spans of waterbeds are a couple years longer; and waterbeds are not as susceptible to microscopic parasites that may trigger asthma, eczema, or allergies.