The difference is much smaller than it appears; add the needed power plant and fuel for the real cost of the weapon system.
Meson J: 1000 Dt + 800 Dt + 800 Dt = 2600 Dt.
Meson N: 2000 Dt + 1000 Dt + 1000 Dt = 4000 Dt.
PartAcc T: 3000 Dt + 1000 Dt + 1000 Dt = 5000 Dt.
So, a Meson N installation is 4000/2600 ≈ 1.5 times as large as a Meson J, hence a ship carrying it is 1.5 times as large and expensive.