not sure this answers your question but like-kind exchanges no longer apply except for real estate. so whatever they gave you for the trade-in is the selling price of the trade-in. if you paid additional cash that's because the value of the new car exceeds the trade in value of the old. so you would pay the difference. your basis for the new vehicle is the trade-in value + the additional cash paid.
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there can be situations where the trade-in value of the old is more than the sales price of the new. in which case you would end up with cash in your pocket. the selling price of the trade-in is its value and the cost of the new vehicle would be the same as the trade-in value of the old less the cash you got
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more complicated situations can arise if the either or both vehicles are/were leased.
