Tell the sales rep that this will in no way turn into a sales skill test vs a buyer skill test. You expect to be treated in a reasonable manner and not as an opportunity to test the sales tactics.
How much is the trade going to cost me. Get a firm price and of course counter with: That is no where near what I'm prepared to spend (finance), thanks and walk out.
OR: How much is the trade going to cost me? Counter with: "that isn't all that far off of what I'm prepared to spend. I'll give you $xxxx.00 if thats acceptable, we deal, if not remember that this is my first, final, only offer so thanks again for your time.