Roughly $1 million, marginal cost (if Wikipedia is to be believed). Each 3 engine Starship prototype conceivably only costs several million.

Compared to what NASA is paying for the SLS engines (over $140 million each), it’s loose change.

Well, one of the aims of the Raptor project is good/cheap manufacturability of engines. They do plan to produce them in massive series.

The really costly phase is probably already done - the design work and early experiments.

Its not that high, because its designed from the beginning to be produced in the 100s per year. Its probably being produced faster almost every other rocket engine (outside of SpaceX own) and maybe some much simpler engines from Russia or China.

They are producing SN100ish by now. A while ago they said 2 million $, already cheap for such an engine. Target is more like 200k$.

So likely its between 1-2M. That would make it only about 6M per flight based on the engine.

Presumably a vast majority of that cost is in R&D rather than manufacturing.