Let's say we base produce 20 hulls for this design. If we go for at least 1 Aft standard launcher, that's 45ish extra Cost over the lifetime of production. If such armament aids in preventing the loss of even one Hull, we'll more than break even on that investment, we'll have saved close to 20 Cost from not having to commission another ship to replace it.
If 1 Aft Phaser is added, the extra Cost over production lifetime jumps from 45 to 80 and we'd need to have saved two Hulls from being otherwise lost for the investment to be worthwhile.
If both are added, the extra Cost over production lifetime jumps to an eye watering 125, and two Hulls being saved from otherwise being lost is needed to barely break even, 3 or more for it to be actual longterm savings.