Every place I have ever lived in the USA, the taxes will be based on the full retail price of the new item you are acquiring. Your credit for the trade in may be applied towards your final bill, but the state (and or city and or county) will want their full tax due on the retail price of the new item.
Some states don’t work that way, but according to online sources, most, including CA do so. When the discount or rebate is coming from the manufacturer themselves, most states do not exempt that portion from taxation since the new product is still being sold at full price. The rebate may be applied to that, but since it is in the form of general store credit, it’s not considered a taxable price reduction.