A fair 1978 price comparison would be between a Base Firebird Formula and a Trans Am WS6 (4WDB) with the optional T/A 6.6. There was 15% between the Base models, and the other performance options listed totalled another 15%. Keep in mind, 1979 saw over 100,000 T/As assembled, many times more than the number of z/28s we'll see.
Volume pays the benefit of lower piece costs. LACK of volume CO$T$...