The trade in value is usually very close to the wholesale value. Dealers buy at wholesale and sell at retail. Way back when I was chairman of a company credit union loan committee the wholesale value used to be at about 70 percent of average retail.
Some libraries have the commercial version of the NADA book which I understand has a wholesale/trade-in value listed.
Dealers play all sorts of games with the numbers but buy at wholesale or less and sell at retail or more is what they do.