Hi just1chancefree,
You can see the criteria I use for memory foam mattresses in post #10 here.
The Sleep Innovations 10" Sure Temp mattress you linked has 3" of 3 lb memory foam. That would be enough for me to eliminate it.
The Lucid by Linenspa 10" memory foam mattress you linked includes 2.5" of 3 lb memory foam which also would be enough for me to eliminate it.
I would not call these “remotely decent” and they would have a high risk of premature foam softening.
You can also read a little more about my thoughts about buying from Costco in post #4 here. In most cases they would certainly not be my first choice when there are other memory foam options available to you that are better quality and value and also have good return policies.
I would also keep in mind no matter where you make your purchase that 3 lb memory foam can soften fairly quickly to the point where the mattress is no longer suitable in terms of comfort or support and if you are on the edge of your range in terms of the softness level that is suitable for you when the mattress is new then this can happen quite quickly (sometimes even in the initial break in period).
There is a list of some of the better online memory foam options I’m aware of in post #12 here and if you are looking at online memory foam choices I would choose between these based on which one was the closest match to your personal value equation (considering both the mattress and the return policy in case you make a choice that isn’t ideal for your specific needs and preferences).
Less than the lowest budget options out of these and you would likely be limited to very low quality materials. All of these will also have the knowledge and experience to give you some good guidance on the phone.
Some of these have mattresses that approximate the Allura in terms of “feel”, quality of materials, or in design (or in some cases all three) that are significantly better value but it’s probably not realistic to think that a mattress in your budget range (assuming it is in the $500 range or less) will “match” the Allura although there will be some that may be “suitable but different” in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences).
Phoenix