Go to Edmunds, go to Edmunds, go to Edmunds.
Go to their "used car" page:
http://www.edmunds.com/used/index.html
Click the link for the "TMV - Appraise a used car before you buy."
Go through all of that, configuring the car with the mileage and the options it's got, and see what Edmunds comes up with a TMV price.
Also, take the link to get the value of your trade-in.
Print those out and bring them to your dealer -- just insist on getting a decent deal.
But, like what other folks have already said, "get your financing first," or at least do some research into it, to know what is reasonable, and what isn't. Also, bear in mind that you might get a better rate for a new car than you would with a used car -- you
MIGHT WELL be able to afford the new car, if you're just talking about monthly payments.
Good luck!