you are only counting labor costs at the end assembly but UAW and other union workers provide labor all along the supply chain and that labor is reflected in the price of parts procured by GM. So if you use your methodology it will appear that the majority of the cost of a car is in materials, not labor, but your methodology is flawed.
Toyota takes about 20 hours to build a car. Give or take a few. GM and TM shared a lot of build techniques, with GM taking a lot more from the agreement, so I assume GM would be well under 30 to build a car. Labor cost, including any benefits, that's another story.