But it can negate the cost difference. There is a crossing point where the price per weapon does indeed make it cheaper to maintain 2 different supply chains. The price difference between AMRAAM and Meteor is something like $700k. If your logistics is costing you more than $700k per missile, you're doing it wrong.
What is that point? You are taking about taking two totally different systems here. Different mission systems, different weapons, all requiring different logistical set ups to both sustain, and modernize and maintain as you hopefully acquire more control over the weapons system over time.
The AMRAAM and Meteor comparison is rather moot here. The AMRAAM-D is a stepping stone for a future missile, while the Meteor is Europe's future missile. Secondly, when you go and train and then look to modernize you will have two procure seperate programs for the same. Same applies to interoperability. You can't simply take your meteor stock and mount it on US fighters, just as you can't take an AMRAAM and son-of AMRAAM and mount it on the Rafale.
So you're saying the IAF should have pursued an all MKI fleet.
No but that is something that has to do with right sizing your fleet. Both the Rafale and the Super Hornet are medium role twin engine fighters and therefore in the same class. Both happen to be capable of being operated from land and sea. Both pretty much serve the same mission set with the French and the US armed forces. We aren't comparing the Rafale to an F-15E, or to a Gripen-C/LCA here.
The reason to pursue commonality is to reduce costs. If 50 A + 50 B is cheaper than 100 A, then saying you should pursue commonality for commonality's sake is foolishness.
It's not always just about cost (but even then you don't know that a diverse fleet or splitting essentially 120 aircraft into two types that have nothing in common isn't more expensive) but also about synergy that you derive from having commonality in the way you train and the modernization strategies that each enterprise pursues.
As another data point, airlines have found that once fleets reach a certain size, the cost impact of maintaining 2 different fleets is minimal. The reason for this is obvious, you can setup a maintenance facility for one type, but after a certain number, you're going to need a second maintenance facility. So if you need a second facility, whether it's for type A or type B is irrelevant. The magic number for airlines was something like 60-90 aircraft. And not just facilities, everywhere along the logistics line there are scaling issues. Granted that usage patterns for airlines and militaries are quite different, but many of the logistics lessons remain the same.
Apples to Oranges. No one that I know of would claim that a small fleet of 57 jets has reached a critical mass where commonality and benefits derived from it are greatly diminished or negated. In fact even if we up this to 120 it still does not make sense (i.e 120 : 57 ratio). Even the USAF with 180+ F-22A's had to consolidate O&S and logistics to bring cost down..the volume was not there to affordably base and operate it like the F-15C's of the past.
Then there are strategic reasons where you can buy into the program in terms of control if you have enough volume to justify this investment. Think 2030 and you negotiate acquiring control or license to allow deeper modernization and inclusion of indigenous capability. It's going to be cost prohibitive to negotiate something like that for 36 aircraft or 57 for that matter. But 100+ and it may look more attractive since it benefits a far larger fleet.