[Alneyan]

The cost is not altered by using mounts, if you keep using the same amount of kt for your weapons. (Say, with 180kt for weapons, you can have either 6 normal APB or 2 massive mounted APB)
- 6 normal APB cost 400 minerals each, so 2,400 minerals in the end
- 2 massive mounted APB cost 1200 minerals each, so once again 2,400 minerals.

The only changes of the *ship* (and ship only, it does without saying that base or units mounts are compulsory. But ship mounts do NOT give range bonuses or accuracy bonuses in the vanilla game) mounts is an increase in firepower and having less, but bigger and slightly stronger weapons. For the sake of completeness, mounts also increase supply usage a tad, if you compare the same amount of kt. But obviously, as SJ pointed out, if your ship is destroyed in a single turn, losing weapons earlier isn't going to change much.