Seems Apple is just too cheap...you can get a fantastic SATA3 128GB SSD for under $100 anywhere...Kingston brand as well as other name brands like Intel or Crucial. A 256GB SSD is about $170. A 512GB is, unfortunately, about $500 but that price will come down very fast.
THOSE ARE RETAIL PRICES. Apple would be paying about 50% (or less!) of those prices with the hundreds of thousands of units they would purchase.
Apple should just skip all the time and effort of "fusion" and plop in SSDs. Use Apple/Intel's "wonderful" Thunderbolt (lots of laughs by me) for external stuff...or a 2TB USB3.0 drive for $100 for all your "storage".
Apple is so worried about having more than 1 logical and physical drive in all the Macs...as if humans can't remember that drive1 is for the apps/OS while drive2 is for all the data (music, vids, documents, movies, etc.).