You'd still have to consider the unfortunate mathematics of orbit-to-surface weaponry. A comparison I've done a while ago told me that my destroyer escorts fire 196 10cm railgun (10/20 damage) shots at 5200 BP for ~26BP/shot, and railgun fighters cost me 51 BP/shot, while it costs PW infantry 0.15 BP for a 15/15 shot, and 0.24 BP for CAP infantry to fire 6 such shots. Of course, a specialized anti-GU design (a beam base?) would be far more economical to field, but I don't think even that could overcome a hundred-fold difference in cost-efficiency.
Your cost math sounds like it comes from before the fighter pods were added. A large fighter (400t) with my current tech can have 6 size 25 fighter pods and each pod (if Autocannon) does 3 shots with roughly 4 times the penetration of PWI and twice the damage. To get a comparable effect from a ground unit, I would have to go up to a MAC on a light vehicle (quite a bit less penetration, but as close as I see) which is almost exactly half the cost of the pod.
A design with armor and theoretically good survivability using micromanagement comes in at 175BP plus 30BP for 6 size 25 pods so 205BP versus 15BP (62.5) for ground vehicles which do similar damage. So yes the fighter is 14 times the cost, but you get arguably simpler logistics and much better survivability (in theory). If you used a Ultra-Heavy Vehicle with Ultra-Heavy Armor and all HACs, you would need 3 to roughly match the firepower and that would be 360BP. So Fighters and FFDs are only as stupid (in terms of cost) as the big ground unit types, well within reasonable costs for the role play value.
With that said, I have no actual experience in ground combat with the new system, or using FFD at all. So my guess that the armor will give me the chance to pull them back to the carrier for repairs may be totally off, thus making the "survivability" completely irrelevant.