If you are going through the costco program, the dealer has to show you costco's discounted price. When I purchased my RX330 (04), I initially went in talking to the salesman that deals with costco customers. However, when he showed me the invoice and the discount, I still thought that I was going to be paying too much. Then the guy told me, "What price is comfortable for you?" I ended up paying 39K before taxes.
Do your homework, research prices, and know your price limit. Again, set up an appointment or talk with the fleet manager or internet managers. They can give you the best deals or discounts. If they haven't ran your credit yet, don't give them that information yet until you've settled on a price that you like. In this case, they're not sure how much you can really afford. Don't let them play with your emotions. Meaning, don't get too attached to the vehicle. If the price doesn't fit, walk away. That's what I did. The salesperson had to come out to my vehicle while I was leaving, and tell me that they can work with my offer.
Good luck! :)