What can I do if I am getting overcharged for a vehicle I purchased?

I traded in my 2006 Toyota Corolla with 71,000 miles to Premier Cars and Trucks in Monroe, LA for $5,500 to put towards another vehicle. The first vehicle I purchased was an Infiniti G35 for around $10,000. I had the $5,500 put towards the car and had the rest financed. The note was $299 for 26 months which I thought was reasonable. The same day I had some misfortune with my roommates moving out on me so I was forced to bring the car back to Premier Cars and Trucks. They surprisingly understood and allowed me to use the $5,500 to buy a car outright without financing it. I ended up getting a 2005 Chevrolet Cobalt with 102,000 miles on it and it had many issues. I was very dissatisfied with the car, but kept it for a few weeks until my roommate decided to stay with me again and help pay some bills. I brought the Cobalt back to Premier and decided to get a 2009 Nissan Cube with 115,000 miles on it. While we were walking to the vehicle to look at it, I was told it was $8,900. I test drove it and decided that I wanted it. I had to get some financed, but I figured in my mind it wouldn't be much to finance so I wasn't worried about it. I went in the office to sign and saw that the vehicle price was $13,750. Then they took my $5,500 off that price. (In my mind the $5,500 should have come off of the $8,900 which means I would only have to finance around $3,400 give or take TTL, etc.) I asked the lady who was doing my paperwork why the price was $13,750 and she told me that is because of taxes... Well I kept the car for about two weeks and after receiving advice from many different people, I decided it was best for me to see if I could bring the car back and just walk off the lot with my $5,500. I went back today, May 23rd, 2013 to talk to them about bringing it back and spoke with the lady who did my paperwork and she then changed her excuse about the price and said it is that high because of bank fees.