making FIX cheaper, since no one uses it. Lower HP

Nobody uses repair bay, because it's not a pre-requisite for anything, and it's expensive. Reducing cost from $1200 to $600.
To compensate for its lower value, its HP are being lowered as well, from 800 to 400. Now it should be killable with an airstrike, which is about right. 

To anyone paying attention: there has been some discussion over whether repairs should be cheaper. I am agnostic on the issue for now. How about we see if people like the repair bay more now and consider using it, before making further balance changes? Open to discussion.
Merge pull request #3048 from psydev/patch-1

making FIX cheaper, since no one uses it. Lower HP