Ooops. Correction.

CTA HAI - around $160
Optimized TB - around $120
Ford Racing wires - around $35
Tranny coller - around $55
Coil pack - around $55
Exhaust - over $500
SuperChip - over $220

I am not counting any suspension mods, any looks or audio mods since that wasn't the question.

I would say that the most expensive mod was Eibach + BAT springs, and the least expensive "Mobile" engine bay sticker