Mattress Forum

While searching for mattress shooping information I stumbled upon this weebsite and its forum and the information available here is amazing and some times confounding for a new member. I did not start looking for a latex bed but upon reading this forum I am gravitated towards it. I looked up options on the common websites mentioned on this forum like flobeds, sleepez etc. I could not understand how the DIY model works. To take an example the Sleep EZ 13000 series King mattress. The current price is $2400 for the All natural 100% latex option. If I build it on my own from the components it is costing even more as each 3'' layer is around $600. I looked up on FBM site and it is coming to $1800 for just the 12'' layers and it does not include any cover etc and have only two ILD choices at each layer. So that brings me to my questions below.

1) What exactly is the DIY model. 

2) What are some of the sources to buy quality material for good price?

3) How much different is the cost for a DIY model vs a finished product like Sleep EZ 13000 mattress? 20%? 30%? 

Some info about what I am looking for in the bed. I am 5'9'' and 200lbs and my wife is 5'4'' 120lbs. Occasionally our two toddlers slip in between us for the night, so to accomodate that I am looking for a Eastern King mattress. We both have some backpain issues as my current 10yr old sealy posturepedic ultra plush is sagging and should be replaced. I convinced my wife that we might need a slightly firmer mattress to get rid of the backpain issues. We are mostly back and side sleepers. I am thinking 3'' 42-44 ILD and 3'' of  34-36 ILD as the core and 3'' 25-28 ILD as middle and 3'' 19-22 ILD as top layer across the board with out any split (specially at the top layer). Any recommendations are appreciated. In terms or material I have seen forum members mention blended latex gives the same quality for a cheaper price but atleast on Sleep EZ site I don't see the difference in price unless its 100% organic. What gives?

Thanks in Advance

Ik.