Cannons really depend on the primary value of their ship, barring double-tap mechanics.

For 3 attack...

Autoblaster (2)

Flechette Cannon (1)

Heavy Laser Cannon (5)

Ion Cannon (1)

Mangler Cannon (2)

Tractor Beam (0)

...With cards like Tie D costing +2 points. Tractor is an opportunity cost no matter what your attack is- either you try for a token that goes away EoT, or you try to do damage. Ion and Flechette sacrifice damage potential for utility- it's not nothing, but you fire it for the utility, and fire your min gun for actual damage.

Mangler and Autoblaster are straight up offensive bonuses, but are either weak (mangler) or hyperspecialized (Autoblaster)

HLC is a straight up extra attack die + ignoring range 3 penalty. It earns an Opportuninst+ level point cost.

For 2 attack, those numbers should be higher.

Making the Autoblaster Cannon cheap enough to fall under the TIE/D classification would certainly be powerful in that narrow window. Were it only 2 attack instead of 3 attack I think it could work well at 3 points on a TIE/D but most everyone else will still need the 3 attack to even consider it at 3 points much less at 4 points. The Autoblaster Turret is only 2 attack for 2 points but it also doesn't have the arc restrictions so an autoblaster cannon with 2 attack shouldn't cost more than 2 and maybe only 1. At 3 points the ABC would cost the same as the Ion Cannon and while it's effective areas is MUCH smaller it can deal more damage more easily.

Who plays Blaster Turret when Dorsal Turret is available? Blaster gets 3 attack @ R2 instead of just 2 but when a focus needs to be spent just to fire it the average damage of 1.5 good results is the same as spending the Focus with 2 dice although the Blaster cost a point more. Lose the focus requirement it can stay at 4 but with it it's not worth anymore than the Dorsal Turret's 3 points and arguably is worth less.

Tractor Beams are about right

Flechette Cannons might be two cheap at one point but they are too expensive at two points for what they do. Either take away the result canceling into one damage or raise the bar for stress so they can at least double stress a ship.

Ion Cannons seem pretty fair from my play experience.

Manglers seem solid

Autoblasters could stand to be four points. Three is too little and would work on TIE/Ds.

HLC seem about right.

Autoblaster Turrets are fair for what they do.

Dorsals are fair

Blaster Turret could/should be three points like Dorsals. If so then which one to take falls more on other upgrades and pilot abilities rather than how it is now where Dorsals are basically just better.

Ion Cannon Turret: I don't know. The range restriction is a pretty big deal when you compare it against the TLT which is only one point more. Maybe it could stand to be four points?

Twin Laser Turret: Seems about right for the power. May be a point to cheap but they seem to be priced pretty well right now.

TURRETS

Twin Laser Turret (6)

This is the only one of the above mentioned cannons/turrets I'd really consider raising the cost to (and I do fly Kanan w/Zeb docked a lot) - its ability to cause damage at high levels without the abilty to modify the dice roll enough makes me think this should be 7 or possibly 8 points; the likelyhood you can defend the attack without taking a damage warrants the increase. Almost is a Palpatine in regards to advoiding damage is almost 0%

lol, FFG doesn't like to give 2 weapons in the same slot the same point cost because one would get played and the other would be ignored. So instead they just keep one overcosted and ignored.

It is no secret that blaster turret is the worst upgrade in the turret category. The requirement to spend focus really sunk it as that was valuable dice modification and if you need to shoot outside of arc they have PWTs. However it is kind of funny because blaster turret is sort of used as the roof for all other turrets. TLT has a maximum of 2 damage and dorsal turret (VCX only) can do up to 3 damage but only at range 1. Still with these different turrets Blaster turret is by far the worse. R4 agromech was origionally designed to help out blaster turret (by trading that focus to a TL) but thanks to overpowerd combos and mechanics creep complicating the standard attack move it was "timed" out to where blaster turret just sinks focus and is not worth it.

As for best Turret, TLT, good thing it is a 6 point but because of its ability to outdamage firepower 3 over time I think it should have cost the same as an HLC.

For worse cannon autoblaster take the cake. Sure there are some that do little to no damage but given the right circumstances and the hit effects those could be great. Autoblaster should have been firepower 4 with the range 1 restriction because most ships with a cannon is primary firepower 3 so at range 1 they would be rolling 4 dice. Put autoblaster on and you sacrafice a dice for your hits being unavoidable but you sacrifice your crits as they can be blocked first.

Best cannon would be the HLC, now again HLC doesn't mean best for all ships (I still think Mangler is the better price match for the Heavy Scyk option). But HLC has the heavy firepower, the range 3 bonus dice nulification. Sure jsut because it knocks crits down to hits doesn't mean it can't deliver hits (rerolls for the win). HLC is the most expensive secondary weapon but considering how much of a meta weapon it was until TLTs arrive it was worth every point.

Cannon and Turret upgrades cannot be compared in price between each other category.

Most ships that can equip cannons are quite well costed currently, so cannons are costing close to their real value.

However, most ships that can equip turrets are heavily overcosted if taken naked (Y-Wing, HWK, K-Wing, ...) so the discount comes with the almost mandatory secondary turret upgrade, to balance it out.
TLT should probably cost about 8-9 points and Ion turret about 6-7 points, and autoblaster turret about 4-5 points, if they costed what they are worth. Instead, they have a -2 or -3 point discount built in to fix those overcosted ships. It also means that all new ship that can equip turrets will come overcosted by 2-3 points.

Of course, Blaster Turret has no justification at all.