This may be impossible to answer, since the data is often not disclosed.
But in an article, about ULA getting RD-180's from Russia it claims:
Each engine reportedly costs between \$11 to \$15 million.
I have never seen prices quoted for a Merlin 1C or 1D, or say an RS-68 (Or RS-68A), an SSME or an NK-33/AJ-26. Comparing different engines in terms of cost could be very interesting.
Of course, an RD-180 has the thrust of 10 Merlins or so, or almost 2 RS-68s or SSME's so probably worth noting common number of units of each used in a typical launcher.