According to Dauntless, 2Torps are equal to 1 Lance. 4Lances are equal to 8 torps. A Lunar replaces its 4 lances with launch bays that can launch 4 torpedo bombers and therefore 8 torps to become a Dictator for a premium of 30pts.
You gain significantly more fire arc and focus, but also a strong possibility of being intercepted and an RO check to do anything at all, but this is also already taken into account in the 30pt premium for regular launch bays - so far no justification for price hike.
Compared to stardard bombers against T2 6+/5+, an 8 torp salvo rolls 7D6 after turrets against facing armour and subsequent targets. whilst the bombers roll 5D6 against weakest armour. This would seem to justify a 12pt price hike, but torp bombers are also easier to intercept as they cannot shotgun - they may only be converted at the start of a subsequent ordnance phase.
Summary:
In absolute power terms, relative to ship based weaponry, there's no reason for a price hike.
In comparison to regular bombers, there may be an argument depending on target type for 5pts per pair of launch bays.