2) Cost
--> B) cost of "customizing a few units"
-----> it is a general rule of mass production...that the more units that are produced of the same exact spec...the lower the cost per unit. For an example...where I work...among some of the products that are made...include tablets and capsules. The production line gets set up for a certain bottle size, with a certain corresponding lid size, the label of a certain size that's gonna go on the bottle gets set up. Then when everything is in order...the production line starts rolling. It takes time...and cost(technicians setting up the production line for that specific packaging size)...so if u make a million bottles...that cost of setting up the production line gets divided over those million bottles. If u make 5 and then change up the production line for a different size of packaging or a different product...then make 10...and so on...
...then that's more cost per unit. This applies to anything and everything that is mass produced...including fighter jets. This is why the cost of the F35 has come down(due to mass production)...if they start adding customizations disrupting the production line...it will go back up. The same holds true for F16 Block 70...the US is mass producing them in one set configuration...if they disrupt the production line to add a different radar for Pakistan's F16s...the cost will go up.