I do not know for sure, but believe that cost is more expensive for IDI than DI. In a DI engine, the combustion chamber is the circular bowl that makes up the top portion of the piston. It is as easy to cast or forge this shape as a flat top, dome top, or any other shape. An IDI pre-combustion chamber is very often a separately cast and machine piece that then has to be integrated as part of the cylinder head. These inserts were typically made out of much harder and more heat resistant metals as they would get much hotter. Think about it, the fuel would be injected in the pre-combustion chamber to initiate combustion, but these pre-combustion chambers would not have the benefit of being cooled by air moving in & out like the rest of the piston/cylinder. I remember that most machine shops would refuse to 'deck' VW diesel cylinder heads because they would not stay flat. They would not stay flat because the precombustion chamber was made out of some hard alloy, and not soft as the rest of the cast iron cylinder head.
