My battlecruiser can fire two cruiser razors and only lose about 3 bars of energy (around 15% energy). A battleship firing two battleship razors takes almost 50% energy. A cruiser firing two cruiser razors takes around 20-30% energy.
Battleship razors, while more powerful, are completely out of whack in comparison with other razors. BS Razors should probably take about 50k-100k less energy to fire, and do slightly less damage.
All in all, the damage-to-energy ratio should be identical between gunboat, cruiser, and battleship weapons in regards to each class of ship.
If a weapon is meant to take up 10% of a battleship's energy (90k), the same weapon type for cruisers should take up 10% of cruiser energy (27k) and for gunboats the same percentage of gunboat energy (I think around 16k). Problem is, as you go up in ship class, weapons become less and less efficient.