Turrets compensate for their immobile nature with vast firepower (about 3x what a cap of fleetships has). They do add to your offensive capacity, directly through drones, and indirectly through your fleet not having to defend.
3x DPS, but less HP. They also cost more energy, and almost twice as much in resources. And while you are correct that they free up units for offense, they cannot contribute directly. I can always move my defensive mobile units to the front when needed, then back when needed (and ususally do). But being unable to move at all, even within system, really hurts the turret's ability to do full damage.
Would you be happy with:
Basic keeps its price and "cheap turret" moniker.
Spider to 2000 (extra cap of snipers +engine damage is still pretty good)
Missile to 2500-3000 (expensive, but better than other turrets on a per-unit basis)
MLRS to 1500-2000 (one of my least favorite turrets)
Flak to 750-1500 (almost-honorable mention, and rarely contribute full DPS)
Lightning to 1000-1500 (same, but are a bit more powerful best case)
Laser to 1750-2250 (bit better than MLRS because it counters bombers)
Counter-whatevers unchanged.
Tractors to 1000-2000
Gravity unchanged (grav effects still quite useful)
HBC unchanged (have you seen their DPS?)
MKIV turrets sound fine, though I would rarely use them.
Apathetic toward drone level adjustment.
Missile turrets are currently about 15-25% more powerful than the other 3 common-type turrets (Basic, Laser, MLRS, Missile). I think they should be nerfed a bit. That way, each of the four is roughly comperable, just with different specialties. (I'd also suggest doing a rename, Basic->Needler, again to match the Guardpost and ModFort/Champion module names).
Then, I ask, if Basic turrets are OK where they are (750K/1500K) doing 470,000 cap-DPS with very useful multipliers against Heavy, Ultra-Heavy and the triangle Artillery...
Why should Missile, Laser, and MLRS all be 50-100% more expensive, while doing as much as 20% less DPS and no better multipliers?
Flak turrets get a lot of... grief... around here, but as long as they can hit 3 targets, they're as good as any other common-type turret. If they can hit all 5, that's 850,000 DPS for the Mk I. Even under FF, that's over 600,000 DPS. Pretty good damage, if you're defending at the wormhole.
If that's fine at 750K/1500K, why does that make the other turrets too cheap?
HBC unchanged (have you seen their DPS?)
Yeah, should have been explicit - HBCs are fine. I'd love to see the Mk II and Mk III cost come down by 500K each, but the Mk II is still a regular unlock for me as is.