Making the total cost of bombers and frigates the same as fighters would be overkill because generally the damage output and survivability of those two are greater than the fighter. I do support making both being made cheaper though. Just not that cheap.
I haven't thought enough about the complex relationships between the triangle ships, but how are the bomber and frigate more survivable then the fighter? The fighter has much higher raw HP. Sure, the bomber has some armor, but rarely does that make a difference in the big scheme of things. The frigate is just fragile overall.
The balance would be much more readily apparent if the bombers and frigates automatically prioritized attacking their bonus targets, because
only then do they prove their worth. In non-micro fleet action, they just get blown away. I thought frigates were completely useless, but after fighting against Z-bombers they finally moved up to "niche, micro managing unit" instead of "wasted resources"
I find myself consistently following this routine through my games in terms of building priorities:
Fighters > secondary fleet ships > bombers*> starships > frigates
*- unless there is a pressing need to bring down a shield or fort. But if there is any other way of removing the shield (golem, secondary unit) I often use that way. Only during the rare, RARE times before I find my first golem do I use frigates unless I am countering Z-bombers
If the price difference between the triangle ships was not so great, they would seem more balanced because they seem to be balanced aside from their cost. The AI also doesn't treat them differently in terms of numbers.