Hi bmbsqd7,
If you are on a tight budget I don’t think I would be looking at 7 or 8 lb memory foam because it is very costly and may not even be necessary.
For a base layer even 1.8 lb polyfoam will normally be fine and 2.0 lb would give you some margin. I would weight your budget towards the comfort layers which are the “weak link” of most mattresses. I personally wouldn’t buy from FBM and I would even question if they carry HR polyfoam but post #4 here has some sources for mattress components and foam that you may find helpful and less than the prices you are mentioning.
In your original post you mentioned you were looking for a full size … has this changed? Full or queen size will of course be less costly than a king (and there often isn’t a lot of price difference between full and queen).
Phoenix