The Falcon 9 has 10 engines. One nine-engine first stage and a single engine second stage.
The Falcon Heavy has 28 engines. Three nine-engine cores and a single engine second stage.
SpaceX should soon be pretty perfectly recovering the first stages and the fairing and other parts. Only the single-engine second stage will not be recovered.
If 2018 production was 400 engines. Then ten Falcon 9 and ten Falcon Heavy could be produced along with extra single-engine second stages.
In 2018 and 2019, SpaceX could produce twenty reusable Falcon 9 and ten Falcon Heavy’s along with 220 expendable second stages.
The Raptor engines are the same size as the Merlin engines.
The SpaceX BFR has 31 engines in the first stage and 6 engines in the second stage. SpaceX should be able to produce ten BFR per year.
How many customers will SpaceX have to drive down the costs? If they have enough customers and markets then they can drive costs down by ten times from higher usage of the relatively fixed capital costs.