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
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