Hi domtiger,
The Sleep Science Latex GL is significantly more than some other similar latex/polyfoam hybrid mattresses or some other latex mattresses that are available online that also have good return policies (although there may be some costs involved) and that have more extensive options to customize the firmness of the mattress after a purchase (by rearranging or exchanging layers instead of returning the complete mattress). Some of them are listed in posts #3 and #4 here.
They don’t list the density of the polyfoam support core but assuming that it meets the foam quality guidelines listed here then there would be no lower quality materials or weak links in the mattress.
There is also more about the different ways to choose a mattress (either locally or online) and how to identify and minimize the risks involved in each of them in post #2 here and there is 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.
You can see more of my thoughts about buying from Costco in post #4 here. The only good thing about buying from them is that you get a free “roll of the dice” because of their return policy so you can try a mattress to see if it’s a good match for you in terms of PPP with little risk (outside of the time spent trying and/or returning the mattress).
Softness and firmness is very subjective and relative and each person experiences it differently so a mattress that feels too firm for one person can feel too soft for someone else depending on their body type, sleeping positions, and preferences or sensitivity and what they are used to sleeping on in the past and are using as a reference point for soft or firm. It would be similar to “sweetness” or “saltiness” where a food that tastes too sweet or too salty for one person won’t be sweet enough or salty enough for someone else. The good news with a Costco purchase (or others that also have good exchange/return options available after a purchase) is that you can try the mattress in person to see how soft or firm it feels to you with little risk (regardless of how soft or firm it would feel for anyone else).
Most foam mattresses (including latex/polyfoam hybrids) generally do best with a firm non flexing support system but there are also some support systems that are available that have flexible slats that can soften up the “feel” of a mattress for some people (depending on the person and on the thickness of the mattress). Any base that has little to no flex will feel similar to any others that also have little to no flex. There is more about the different types of support systems and some good sources for each of them in the foundation post here.
Phoenix