Here. I did some calculations.
Matter Converter produced 50.000 Energy and costs 200M/s.
So 250 Energy = 1 Metal/second
CapUpkeepCost=CapEnergyCost/250
Mark I Fortress
DPS: 8.000
Cap DPS: 40.000
Total DPS: 40.000
Energy Cost: 90.000
Cap Energy Cost: 450.000
Cap Upkeep Cost: 1.800 M/s
Total Upkeep Cost: 1.800 M/s
Knowledge Cost: 3.000
Total Knowledge Cost: 3.000
Total DPS/Total Upkeep Cost: 22,222
Total DPS/Total Knowledge: 13,333
Mark II Fortress
DPS: 18.667
Cap DPS: 74.667
Total DPS: 114.667
Energy Cost: 120.000
Cap Energy Cost: 480.000
Cap Upkeep Cost: 1.920 M/s
Total Upkeep Cost: 3.720 M/s
Knowledge Cost: 4.000
Total Knowledge Cost: 7.000
Total DPS/Total Upkeep Cost: 30,824
Total DPS/Total Knowledge: 16,381
Mark III Fortress
DPS: 33.333
Cap DPS: 100.000
Total DPS: 214.667
Energy Cost: 180.000
Cap Energy Cost: 540.000
Cap Upkeep Cost: 2160
Total Upkeep Cost: 5.880
Knowledge Cost: 5.000
Total Knowledge Cost: 12.000
Total DPS/Total Upkeep Cost: 36,507
Total DPS/Total Knowledge: 17,888
Super Fortress
DPS: 200.000
Energy Cost: 300.000
Upkeep Cost: 1.200
Total DPS/Total Upkeep Cost: 166,667
vs Mark III Fortresses: Cap DPS/Upkeep Cost: 99,382
Mark I-III Fortresses and Super Fortress with Zenith Power Generator
DPS: 414.667
Energy Cost: 840.000-600.000=240.000
Upkeep Cost: 960 M/s
Total DPS/Total Upkeep Cost: 431,944
Mini-Fortress
DPS: 800
Cap DPS: 1.600
Energy Cost: 9.000
Cap Energy Cost: 18.000
Cap Upkeep Cost: 72 M/s (The game should pay ME for using these things)
Knowledge Cost: 1.000
Cap DPS/Cap Upkeep Cost: 22,222
Cap DPS/Knowledge: 1,6
Mark I Heavy Beam Cannon
DPS: 1.200
Cap DPS: 14.400
Total DPS: 14.400
Energy Cost: 1.650
Cap Energy Cost: 19.800
Cap Upkeep Cost: 79 M/s
Total Upkeep Cost: 79 M/s
Knowledge Cost: 500
Total Knowledge Cost: 500
Total DPS/Total Upkeep Cost: 182,278
Total DPS/Total Knowledge: 28,8
Mark II Heavy Beam Cannon
DPS: 3.600
Cap DPS: 28.800
Total DPS: 43.200
Energy Cost: 2.500
Cap Energy Cost: 20.000
Cap Upkeep Cost: 80 M/s
Total Upkeep Cost: 159 M/s
Knowledge Cost: 2.500
Total Knowledge Cost: 3.000
Total DPS/Total Upkeep Cost: 271,698
Total DPS/Total Knowledge: 9,6
Mark III Heavy Beam Cannon
DPS: 10.800
Cap DPS: 43.200
Total DPS: 86.400
Energy Cost: 5.000
Cap Energy Cost: 20.000
Cap Upkeep Cost: 80 M/s
Total Upkeep Cost: 239 M/s
Knowledge Cost: 3.000
Total Knowledge Cost: 6.000
Total DPS/Total Upkeep Cost: 361,506
Total DPS/Total Knowledge: 14,4
Mark IV Heavy Beam Cannon
DPS: 58.000
Cap DPS: 58.000
Total DPS: 144.400
Energy Cost: 20.000
Cap Energy Cost: 20.000
Cap Upkeep Cost: 80 M/s
Total Upkeep Cost: 319 M/s
Knowledge Cost: 3.000
Total Knowledge Cost: 9.000
Total DPS/Total Upkeep Cost: 452,664
Total DPS/Total Knowledge: 16,044
Mark I Laser Turret
DPS: 112,5
Cap DPS: 5.400
Total DPS: 5.400
Energy Cost: 400
Cap Energy Cost: 19.200
Cap Upkeep Cost: 76,8
Total Upkeep Cost: 76,8
Knowledge Cost: 0
Total Knowledge Cost: 0
Total DPS/Total Upkeep Cost: 70,312
Total DPS/Total Knowledge: -
Mark II Laser Turret
DPS: 225
Cap DPS: 10.800
Total DPS: 16.200
Energy Cost: 400
Cap Energy Cost: 19.200
Cap Upkeep Cost: 76,8
Total Upkeep Cost: 153,6
Knowledge Cost: 750
Total Knowledge Cost: 750
Total DPS/Total Upkeep Cost: 105,468
Total DPS/Total Knowledge: 21,6
Mark III Laser Turret
DPS: 337,5
Cap DPS: 16.200
Total DPS: 32.400
Energy Cost: 400
Cap Energy Cost: 19.200
Cap Upkeep Cost: 76,8
Total Upkeep Cost: 230,4
Knowledge Cost: 1.250
Total Knowledge Cost: 2.000
Total DPS/Total Upkeep Cost: 140,625
Total DPS/Total Knowledge: 16,2
Mark IV Laser Turret
DPS: 450
Cap DPS: 21.600
Total DPS: 54.000
Energy Cost: 400
Cap Energy Cost: 19.200
Cap Upkeep Cost: 76,8
Total Upkeep Cost: 307,2
Knowledge Cost: 1.500
Total Knowledge Cost: 3.500
Total DPS/Total Upkeep Cost: 175,781
Total DPS/Total Knowledge: 15,428
EDIT:
Conclusion:
It seems to me that Turrets and Fortresses are quite well balanced. Though Fortresses are a viable option only if the player has a Zenith Power Generator. They're also only for small/medium sized empires (+-7 planets including Homeworld). For small/medium sized empires because Fortresses have galactic cap and large empires have "too many" planets connected to the AI planets so the player would run out of Fortresses. Of course you can do gate raiding but you can't really control where the CPAs and Exos come from. Fortresses should be combined with Missile or Sniper Turrets to counter Bombers. I'd choose Missile Turrets over Sniper Turrets because Missile Turrets shoot.. missiles.. Missile Turret shoots 3 missiles and if the target is destroyed before all missiles hit the missiles will find another target. So Missile Turrets aren't as likely to overkill an AI ship. Less damage wasted. Also they have damage multiplier vs Neutron so they will help you to get rid of those pesky Zenith Starships. And since you're (probably) going to use Economical Command Stations Munitions Boosters are a very strong starting ship.. so you might want to get Mark II Munitions Boosters and Mark IV Fighters (and Bombers too.. always Mark IV Bombers (maybe)). Last but not least.. Fortresses got an indirect buff when the Carriers were changed. Thanks to these new Carriers Fortresses aren't as likely to face Bombers because the new Carriers can't be destroyed like the old ones. Fortresses are more likely do their full damage because damaging the Carrier damages the ships inside the Carrier.. so even if those ships were Bombers the Fortresses are going to do their full damage. Now don't get me wrong.. overall the new Carriers make the game harder but it still was a small indirect buff to Fortresses.
Turrets are OP early game (but not in a bad way. not too OP) but become balanced as the game progresses to mid/late game. As you can see Laser Turrets' cost effectiveness decreases as the Mark level increases. In my opinion it's a fair trade off for the per planet cap and low Knowledge costs. Turrets are at their best with Economical Command Stations and for large empires or for small/medium empires with beachheads. If you're running a small/medium empires with very few of no beachheads Military Command Stations are the better choice. Turrets are the #1 choice for low AIP games.
I'd say Heavy Beam Turrets aren't as OP as people think. They're quite expensive in term of Knowledge but are very cost effective in terms of DPS/Metal. They also have galactic caps.. so they're good for small/medium sized empires. Thanks to their metal-cost-effectiveness they're at their best when combined with Mark III Military Command Stations. No Economical Command Stations needed.
Last but not least: Mini-Fortresses are bad.
EDIT 2: Once again.. I don't understand why people complain that per planet Turrets make the game too easy. I mean I understand where you're coming from but you can always increase the difficulty level unless you're already playing on 10/10. To me 9 difficulty AI seems pretty much lobotomized so I'm not surprised some find the game easy. Also as I said in my conclusion Turrets are OP early/mid game and become less cost effective at mid/late game and as the Mark level gets higher.
Finally.. if you think Turrets are too overpowered.. take a look at the stats in this post and tell me your opinion how (normal) Turrets compare to Fortresses and HBCs.