"Trains" ran a several (three?) part article on the basics of diesel locomotive technology, back in the 1970s. At that time the rough estimate was that, by cost, a diesel-electric was about one third the Diesel engine, one third the electrical stuff, and one third everything else. Since a new (main line) locomotive these days will set you back. ??? maybe three million dollars? my guess would be that a new engine (assuming the builder was willing to sell you one) would be about a million.
But, also from ancient history... periodic overhaul of the engine and of the rest of the locomotive didn't take the same amount of time. So railroads with large fleets, when a locomotive came in for a major overhaul, would remove the engine and deal with it and the rest separately: when it was time to put an overhauled locomotive back in service, they would install an available engine (one the same sort), which might not be the one that had come out of the locomotive at the beginning of the overhaul. I believe the New York Central had this policy for locomotives with 1600 hp Alco 12-244 engines (so: FA-2, RS-3) since they had a couple of hundred. On the other hand, the NYC had only a handful (7? 9?) of locomotives with 12-251 engines (RS-11), so this system was not set up for them: when an RS-11 came due for overhaul, if the rest of the unit was done before the engine, it would just stand, engineless, until the original engine was ready for re-installation. (And, no, I don't remember where I read that. My source talked about the Alcos: I assume that the procedure would have been the same for the hundreds of units with 16-567 engines.)