My $0.02 on this:
1. All grain is cheaper, but if you don't want to do AG, don't. Many have pointed out that it can take a bunch of beer to recoup the hardware upgrades.
2. Any kit can be re-produced from separately purchased ingredients. Save the recipe sheets from your kits to see what they generally use.
3. Any ingredient can be bought in bulk to save 10-50% on the ingredient. Some items store better than others. DME sealed up tight and Hops in a freezer last a LONG time, LME in an opened can or Milled Grains in a paper bag will not last long.
4. Storing bulk anything probably adds overhead cost. You'll need sealed buckets/bins/jars/etc.
5. Cost isn't the only reason to consider buying bulk, having ingredients already on hand means you can have impromptu brew days.
You can certainly save money building your own kits, although you may have to shop around for ingredients. I suggest you take your favorite kit recipe and spend a bit of time shopping and pricing out buying the ingredients separately. Include the option of buying two or three batches worth at a time to see how much savings on shipping you can get.
I set a goal for myself of $30 or less in ingredients per 5g batch. By doing BIAB and saving yeast, I can brew IPAs instead of Cream Ales at that price point.
