If the price(s) of "already-bought" item(s) is(are) not subtracted from the kit price then anyone who buys a kit with "already-bought" items pays more than the cost of the kit.

look:

hull: 15000

turret: 10000

paint: 25000

kit: 50000

after 50% disc: 25000

but look:

hull: 15000 - already-bought

turret:10000

paint 25000

kit: 50000

after 50% disc: 25000 + 15000 = 40000 because 15000 was ALREADY SPENT