Hi avgenie,
Having read through the new ETOPs plan for 3 or 4 engine aircraft, it seems to me that most of the responsibilities will be on the operators (airlines) that fly these long over water routes. Route planning and diversion planning becomes a more demanding issue than what it has been. I don't see it as a big cost item for an engine manufacturer unless there are unforseen problems that develop in revenue service.
For new aircraft/engine combinations, ETOPs is generally applied for and demonstrated during the certification phase. Since most airlines establish contracts for the aircraft and engines separately, I would bet there will be penalty clauses established should either fall short and the contracted ETOPs rating is not achieved or becomes reduced because of problems. This is where the bottom-line of either manufacturer could be impacted depending who has the problem and how long it takes to resolve it.
There is another large customer today, leasing companies. They buy airplanes and engines and lease them to various airline. Since these deals are very complex and vary considerably it is hard to tell how this change will affect them.