[Suicide Junkie]

Ok, here's my math on it: Since halves are confusing, I will take 40% core and 60% weapons/shields/etc.

You build the ship and start paying maintenance early, but you get it into service early too, so there's no net change there.

So, the plan is to build 40% of the ship now, and to retrofit 60% of the components on later.

You pay 40% of the maintenance while it flies to the training centers near your front lines. This is pure discount since a full ship would take the same time to get there.

Once there, you sit and train for a couple turns at 40%. More savings.

Now you retrofit over the training world. You are adding on components worth 60% of a full ship. (This will take multiple retrofit steps). You pay 120% of the build cost of the 60% added now.
No components are removed, so that cost is zero.
In total, you pay (1.2 * 0.6) 72% of the cost of building the whole ship at once. Add on the 40% you already paid for, and the ship cost you a total of 112% of the normal cost.

While the components are repaired, you finish the training at the regular-price maintenance.

So, we have...
Extra costs: 12% more per ship.
Savings: 3/5ths of normal maintenance for 2 to 8 turns or so, depending on distance from the training center and training rate.

If you have maxxed out your maintenance reduction and are paying the minimum 5% of hull cost per month, your savings turn out to be:
3% of full hull cost per month.

After only 4 months, you break even!
Without maxxed maintenance reduction, the retrofits give you a net resource savings sooner.

If you have just one training center at 3% per month, your ship will sit there for 7 months training. If it takes 2 months to repair the components, even ships built in the sector see a small net savings from the 5 months of reduced maintenance.