If I want to buy a car for 390.00 a month for 5 years at rate of 6.5 percent what should the highest I pay for the car

Depends on how much you put as a down payment for example car is 19,800 then add the tax and destination fee it comes out to roughly 21,698 (I am using PA's tax of 6%) with 2,000 down it will be around 399.