Investigating? GTN farmers were making too many credits purchasing Cartel Packs and selling it contents at inflated prices. In your unholy fear of inflation you 'corrected' the market by raising repair costs, sort of like a release valve. When anger is sufficient and the head-less chicken you consult for your economy give the right signs, you'll lower back down for a few months and repeat.
It isn't a great mystery Clouseau.
Huh? (Facepalm) Pack sales on the GTN don't create credits in the game, they only transfer them from one person to another.