Learn me: From a cost to manufacture perspective, should bigger displacement in a family cost more?

Dumb question time.

I've been looking at Porsches lately. The 986 Boxster and the 996 Targa and Carrera cars all had an M96 engine. Porsche designed the engine to support several displacements, and according to one review I read, a four cylinder variant. When Porsche builds a 2.5l Boxster with ~200hp, or a 996.2 X51 3.6l with ~380hp, how much of that is stuff that should cost more? I know there are other Porsche engines, like the M97, and other engines from other manufacturers that I could ask this about. For example, the GM 60 degree came in a bunch of displacements, and Fiero people are putting 3.4 Camaro engines to replace their 2.8.

So, I'm asking, how much more money did it cost Porsche to make a 2.5 than a 3.4 or a 3.6 M96?

For Porsche, the biggest factor in the cost of an engine is how many they make.  Yes, there's the cost of development- which will be mostly the same price if nothing is carried over- all the same development must take place, with prototypes and cad, yada, yada, yada.  Most large OEM's spend about a $1B for each clean sheet engine.  And given how things have developed over the years, I actually think that price has stayed roughly the same for the better part of the last decade.  The 3.4 and 3.8 may share a ton of parts, but the 2.5l may be a little small.  Of course, the more shared parts, the cheaper.  But it would be hard to have a single block for 2.5-3.8l 6 cyl engines.

Anyway- that part isn't the expensive part.  It's running the engine line.  Which ever has the most volume ends up being the cheapest- especially if they have roughly the same parts and the same complexity.

Having the 5.0l V8 in a pick up is the only reason the Mustang has a V8.  Even if there are only 300k put in trucks every year, that's enough to not make it tough for the Mustang.

Porsche really never gets into the volume discount amounts- just don't sell enough cars.  The 2.5 goes in the Boxter, too, right?  Huge deal adding that.

As I see it.

I don't know ANY specifics about the Porsche but in general you would have a different rotating assembly. ( I think 2.5 to 3.6 is likely a stretch on bore alone in a 6 cylinder)

Likely different valve train components, at minimum bigger valves to take advantage of the bigger bore but possibly a completely different head design.  Different camshafts.  Different intake design.  Supporting fuel system components.  Beefier transmission?

I could be and likely am wrong about most of this but it should be in the ballpark.

On the flip side.  Not a ton changes on a LS engine between a 4.8 and a 6.0.  The 7.0 LS7 is substantially different though.

A couple things:

If the engines have the same number of part numbers and the same quantities, then whichever one weighs less will likely be cheaper.  Unique part numbers are the enemy.

As Alfa said, volume discounts are where its at.  As Alfa pointed out, the volume isn't there for porsche.  Given that, I would say, see the point above.  If you can make a bigger displacement version of the engine with the same number of parts, it will cost pretty much the same.

All of that said, the bigger engine makes more power, which causes more abuse to the rest of the car/drivetrain and thus requires further testing/engineering/etc. there driving up the cost.  This will not be insignificant.

That used to be de riguer for all manufacturers. The small block Chevrolet went from 262 to 400ci, and as a counterpoint the much smaller company American Motors built their gen 2 V8 from 290 to 401ci. It's easy to design the basic block and engine assembly line to accommodate multiple displacements from the get go. 

I'm not positive but my understanding was that the m96 (2.5L to 3.4L) all used the same block casting, with the only difference being the cylinder bore. The air cooled engines used the same case halves from basically the first 6cyl engine through to the end of the line 3.6L version, by way of continually throwing bigger cranks and cylinders at them. Which was how the 3.6 ended up with paper thin clearances between the big end of the rods and the oil pump... they just were running out of room in there.

The other factor here is how much can I mark it up. The larger engine may cost more but I can also get a bigger margin out of it.