Hi lagreca,
In most cases an all latex mattress does best on a solid non flexing foundation with slats that are 3" or less apart but some manufacturers will use an active box spring or flexible slats under the mattress as part of the design of the sleeping system which will add some flex and give under the mattress. This generally works best with thinner mattresses because if they are too thick then the effect of the box spring or flexible slats is reduced or minimized. If you test a mattress with an active box spring or a flexible slat foundation and it works well for you in terms of PPP (Posture and alignment, pressure relief, and personal preferences) then they should be purchased as a set because putting it on a non flexing foundation will change the feel and performance of the mattress.
The prices you are mentioning are on the high side and I typically use the prices for the online manufacturers listed in post #21 here as a “value reference”. I would consider a “local premium” in the range of about 20% - 25% or so (in an apples to apples comparison) to be roughly “equivalent value” to make up for the higher risk of an online purchase that you can’t test first. If the difference is more than that I would consider an online purchase more seriously depending on your comfort level with any additional risk of an online purchase and on the criteria of your personal value equation that are most important to you.
Out of the three local options you mentioned it appears to me that the Escondido Mattress Factory would be the better local value but of course “value” also depends on how well it matches your specific needs and preferences in terms of PPP and any options you have for making comfort adjustments if this becomes necessary.
The Costco Sleep Science 9" talalay latex mattress also seems quite high to me for an online purchase and many of the online manufacturers in the list I linked make a similar mattress with 9" of latex and a wool quilted cover at significantly lower prices and also offer more customized comfort and support options. In some cases they also provide the ability to re-arrange or exchange layers and change the comfort or support after you have slept on it if you make a choice that is less than your ideal. They will also provide you with much better guidance than the employees at Costco who usually have little knowledge of mattresses or how to make the most suitable choice.
When you are looking at latex mattresses the most important information is to know the thickness and type of latex in the layers (Talalay or Dunlop) and the blend of natural and synthetic rubber that was used to make it so you can make “'apples to apples” comparisons with other latex mattresses. The cover materials will also make a difference in the cost and unquilted stretch covers are typically less costly than covers that are quilted with wool (which is often used to pass the fire regulations and can improve the temperature regulation of the mattress although it may also slightly reduce the compression of the softer latex layers below it).
Phoenix