OP,
I find it hilarious how dealerships routinely price their used cars at the same or higher price then their new cars. The Mazda 6 you are interested in is two years old and already has 27K miles on it. Edmunds lists many diff styles of the car and it seems that the going price runs from $14K to $16K. The price of gas is causing the resale value of lower MPG cars down. Try selling an Expedition right now. It's more like giving them away. The Mazda 6 gets about 20/27 which might depress the price of the car as we move towards more fuel efficint cars.
My suggestion is to start in the low $14K range and when they scoff at the offer ... walk. It's not like people are lining up to buy these things.
BTW, I bought a Protege in 1999 (new) and my wife loved it for 8 years. It averaged 35 MPG between city/highway and required only about $500 in repairs over the time we owned it. One of the BEST cars we've ever owned. Foe your sake I wish the 6 got better MPG ...