Hi Harriett,
You may not be making “apples to apples” comparisons based on “mattress only prices” which could be part of the reason for such a large price difference. One mattress could using blended Talalay and the other could be using 100% natural Talalay, there could be differences in the cover and quilting which can be a significant part of the cost of a mattress, or you could be comparing a “set” vs a "mattress only and a real box spring (with springs that flex) can be significantly more costly than a foundation (that doesn’t flex at all).
Even if you are making apples to apples “mattress only” comparisons though these types of differences between similar mattresses are fairly common in the industry because different manufacturers can have different costs and pricing structures (see post #14 here). This is one of the reasons why it’s so important to find out what is inside any mattress you are considering (see this article) so you can make better comparisons between mattresses and especially between your finalists based on the materials inside the mattress and all the other parts of your personal value equation (see post #46 here) that are most important to you and which may affect the cost of a mattress purchase.
There is also more about the most important parts of the value of a mattress purchase in post #13 here that can help you make more meaningful comparisons between mattresses.
The mattress shopping tutorial also includes a list of the members here that sell mattresses online (in the optional online step) and many of these make latex mattresses that use different types of latex with a wide range of designs, features, options, budget ranges, and exchange or return policies that can also act as a useful “value” reference point as well.
Phoenix