69 shelby gt 350 price with a 429 cobra jet?

There's no such thing as a 1969 Shelby GT350 with a 429 Cobra Jet engine. The 429 Cobra Jet was an engine option for Ford's production cars like the Mustang, but not the Shelby GT350. The GT350 used smaller, high-performance engines like the 302 or 428 Cobra Jet. A 1969 Shelby GT350 with a 428 Cobra Jet would be extremely rare and valuable, but a 429 was never factory installed. Any car claiming this would likely be a highly customized vehicle, and its value would depend heavily on the quality of the conversion.