I don't know if the current users of iPhone will take this as a number of them are music producers (or enthusiasts) and their current very expensive hardware won't be compatible with the new iPhones.
Knowing Apple, I'm going to say $30 optional accessory.
The old to new magsafe adapter is only $10, so maybe I'm being too hard on them. On the other hand, their Lightning to HDMI or VGA adapters currently go for $50.
Those $50 adapters have a microprocessor in them to map/convert the eight pins to whatever signals are needed. Lightning does not have an analog audio channel, so I'd bet this adapter would also need a (simpler) microprocessor and the attendant cost. Your $30 guess sounds right, and Apple probably won't even include an adapter as long as they include Lightning or Bluetooth earbuds. People who are fussy enough about headphones to bring their own will pay the additional $30.
Yep, I'm assuming it comes with Lightning earbuds and if you want anything else you're on your own.
Bluetooth earbuds would be a pretty hard sell if they come out and say "We're changing to Lightning for better audio quality. Here, have some bluetooth earbuds with worse audio than your old wired ones."
EDIT: How small can you make a decent DAC anyway? All the audio ones I've seen are way too big to put on an adapter cable, but that might be driven by the optical and RCA connectors more than the conversion hardware.
Right, I think it's more likely they do Lightning earbuds. 1) reinforces the concept, 2) doesn't have the fuss of another thing to charge, 3) Apple can do them cheaper than anyone else if they don't have to pay a licensing fee.