Hi vade11,
Just in case you haven't read it yet ... the first place I would start your research is the
mattress shopping tutorial here
which has all the basic information, steps, and guidelines that can help you make the best possible choice ... and know how and why to avoid the worst ones.
I feel like I am lost in a sea of crappy or too expensive mattresses. FIrst off. My husband and I are in our early 20's. He works a labor intensive job and I have back pain from a bad epidural. We both started having back pain and it finally dawned on us that it was from our mattress. Then it started sagging. We bought the mattress 3 years ago from an outlet store for pretty cheap. It's a king size Beauty Rest. I just walked in and bought a mattress thinking "It's name brand. It will be great" No. Just no.
Unfortunately most of the major brand mattresses use lower quality materials in the comfort layers of their mattresses which are a weak link in the mattress and much more likely to soften or break down prematurely. I would tend to avoid major brands or any mattress where you aren't able to find out the quality and durability of the materials inside it (see the
guidelines here
).
I have read a bunch of info (here and on sleeplikethedead) and looked at tons of mattresses online. I have come to the realization that for a firm mattress that will last longer then a couple years, latex seems to be a top choice. The problem with that is our budget doesn't seem to allow for a latex mattress. I understand that it's an important purchase and you get what you pay for to some extent but I can't pay with money I don't have.
Our budget is around $500-700, not including a foundation or bed to set it on and a protector. I'm feeling like it is impossible to find something at that price. I know that we absolutely want to avoid a memory foam that less then 4lb density.
You are certainly in a lower budget range but there are better and worse quality options available in every budget range and it's just as important to make sure that you choose higher quality and more durable materials in lower budget ranges as it would be in higher budget ranges. Some of the better options or possibilities I'm aware of that are available in your budget range are listed in
post #4 here
. You are just inside the budget range where a latex/polyfoam hybrid is a possibility as well.
The other major factor in our choice is that we bed share with our son. He is almost 2 years old. But we also plan on having more children within the next few years. I prefer to be able to co-sleep because getting in and out of bed all night to nurse every 2 hours is pure hell. Will a foam mattress cause a baby to roll toward us like on an air mattress? I've tried a few memory foam mattresses or toppers but never with a little one present. With co-sleeping we also really need a king size bed. Foot to moms face seems to be a favorite position for our son and I will seriously never sleep again if we downsize.
Post #2 here
includes some guidelines for children's mattresses and links to some of the better forum posts and topics about mattresses and children but If you are planning to co-sleep with a baby or an infant I would tend to avoid memory foam completely (see
post #2 here
).
Air displaces under pressure so it would tend to have more "roll together" than a foam mattress but it will also depend on the specifics of the foam mattress and the firmness of the support core and the thickness and firmness of the comfort layers. You will sink in more to softer mattresses which will create a deeper "valley" that your baby can roll into if they are close to you. It will also depend on the specifics of the material and how "point elastic" it is (how well it conforms to the shape of the body without affecting the wider area around where it is being compressed).
The Dreamfoam 12 in 1 customizable mattress seems to be a good choice for us but I'm not sure if I feel this way out of frustration or fatigue. LOL. I'm just a tired mom 
The Dreamfoam certainly uses high quality and durable materials for its budget range (2 lb polyfoam) and it also has good options to customize the firmness of the comfort and support layers after a purchase so it would certainly be one of the options that are available in your budget range that would be well worth considering.
There is more about the different ways to choose a suitable mattress that can help you identify and minimize the risks involved with each of them in
post #2 here
and 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 in any budget range.
There may also be some good options available to you locally and if you let me know your city or zip code I'd be happy to let you know about the better options or possibilities I'm aware of in your area.
Phoenix