I've heard that the Merlin 1-c engines are about $1M a piece. And that SSME's run $50M each at the current production rates.
Hard to verify pricing for components, especially for SpaceX, as they do so much in house. Who outside of the company knows what the actual production costs of each part are? Hmmm, perhaps we can estimate the max possible cost of each engine based on launch prices and the assumption that SpaceX is not taking a loss on each launch.
A Falcon 9 launch costs $54M, and has 10 Merlin 1c engines. I'm going to ignore the cost differences between the upper stage (vacuum) and lower stage engines. If every thing else (fuel, lower & upper stages, facility lease, profit) were $0, each engine would cost at most $5.4M. In fact, looking at the announced pricing for Falcon Heavy, $110M max, with 27+1 engines, you're looking at less than $4M an engine, max.
Given the costs of the rest of the launch, and number of engines (production scaling efficiencies) involved, I don't think that a $1M per engine estimate is too far off. That puts engines at 25% of the launch costs, and I'm OK with that estimate. I know that the Shuttle SRB's are a higher percentage of the cost of a SLS, but those are an outlier. You can buy 4 Atlas CCB's (with 8 engines) for the price of 1 SRB. Given that pricing, I'm not sure that any $10M engine out there has 10x the thrust of a Merlin 1c.
So SpaceX is probably good with the whole multiple engine thing, at least on price.