Hi Todd,
For the cost difference:
One way you might look at it is by man-hours expended per engine overhaul. They have records for this for every month of the war from Jul 43 through Aug 45 in the Army Air Forces Statistical Digest of WWII, Table 115.
The two engine types you asked about were the Merlin (or V-1650) and the Allison (or V-1710).
For the V-1650:
Man-hours started out in Jul 43 at 462 hours.
For the entire war by month: High: 592 man-hours; Average: 320 man-hours; Low: 190 man-hours.
For the V-1710:
Man-hours started out in Jul 43 at 376 hours.
For the entire war by month: High: 376 man-hours; Average: 191 man-hours; Low: 117 man-hours.
Let's be fair and acknowledge that the average comes down as the workers get familiar with the engine. So, let's use the average: On average, it took 320 man-hours to overhaul a Merlin and 191 man-hours to overhaul an Allison. So, they can overhaul an Allison in 59.7% of the time it took for a Merlin.
I worked at a shop that overhauls Allisons for almost 4 years. In that time, we did some work on a couple of Merlins. Everything in a Merlin took longer, not just because we weren't as familiar with the Merlin, but also because the design is harder to work on in general (takes longer). As an example, in service you have to tighten the cylinder hold down nuts every 25 hours of operation. On an Allison, the cylinder banks are torqued down to 1,100 foot-pounds and never have to be touched until overhaul. Valve seats are another difference. You screw in the Merlin valve seats until they snap off the stud. Allison valve seats are permanent and are harder and more easily ground. Merlin valve seats are not so easily ground and are more likely to be replaced and than ground. I'd say that manufacture of a Merlin would take about 20 - 30% longer for machining and 40% longer for assembly.
So, let's try an example using 25% longer for manufacture. Say we can make an Allison V-1710 with 40 hours of casting, 300 hours of machining, and 50 hours of assembly. The casting should be very slightly longer for the Merlin, so it might come in at 50 hours casting, 375 hours of machining, and 70 hours assembly. The Allison takes 390 hours total and the Merlin takes 495 hours total.
Now, I pulled those Allison numbers above out of my hat (and by extrapolation, the Merlin numbers as well), but the percentages won't be too far off. So, my estimate is the Merlin takes, in total, about 27% more time and cost to manufacture, give or take a bit, and it takes about 40% more time to overhaul a Merlin when compared with an Allison overhaul.
Here's the thing, in my experience, the Merlin is as reliable as an Allison in field operation (after the initial Allison faults were ironed out ... took about 9 months or so), but the Allison runs longer before an overhaul is needed in non-turbo operation (P-39 / P-40 / A-36). I say non-turbo operation because today (my experience), almost all Allisons are operated without the turbochargers. If you think it might be hard to find parts for an Allison V-1710, try finding parts for an 85-year old turbocharger! If you needed high-altitude performance, the Merlin was your better choice for WWII. If you were flying in the PTO or Med, either one would do fine. The Allison cost was less to manufacture and overhaul, but it wasn't really a factor for military operations since the units didn't pay for aircraft ... they operated them. The Merlin used slightly more fuel than the Allison but that, too, was of little consequence because they had enough fuel for operations.
In service, there was little to choose between them once the initial problems with the Allison were ironed out, unless higher altitudes were required. If they were, the Merlin was the choice if there WAS a choice. Mostly, there wasn't a choice. If your unit flew P-38s, you generally didn't have the option to trade them in for P-51B/C/D/Ks. The USAAF made that choice for you, and U.S. units didn't GET Merlins until we started accepting P-51B/Cs, so we are talking after the summer of 1943 to come off the assembly line and late 43 or into 44 before our guys were flying Merlins overseas, at least in P-51s. So, we only had about a year and a half of time when we were operating both engines in numbers. The Brits were operating them together far longer than we were.
Second, why would you believe the Merlin was a double overhead cam? Download a pdf manual from this forum and check it out to be sure.