Yes, you got ripped off, you can try to dispute it with the dealership but you are bound by the contract to pay what was agreed upon during signing at their finance office.
It is imperative that you do your research before going into the dealership. You have an array of online tools to assist you in determining a fair price for both parties. KBB will probably be your best overall resource as far what what you should pay. Other factors such as aesthetics, maintenance, vehicle history, should also be researched online and discussed. As a part of this you should ask a friend who is mechanically inclined to help you inspect the vehicle before purchase to identify any discrepancies. This car could be a non-CPO vehicle, with that being said there is no manufacture's warranty. That's "if" you purchased it from a Infiniti dealership the below material is valid.)
Certified Pre-Owned Cars
"Only Infiniti models less than four years old and with under 60,000 miles qualify for the program, and then they must pass a rigorous inspection."
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www.infinitiusa.com/pre-owned/overview.html
Good luck in getting your money back, you can try to dispute directly with the dealership. If they do not stand behind their cars and give you the run around that gets nothing done. You should contact the Nissan Corporate to file a complaint and go to BBB (Better Business Bureau) and file a complaint. Last resort is to hire a lawyer but it will be long drawn out battle due to the finance contract you signed. (That's "if" you purchased it from a Infiniti dealership the material I provided is valid.) Remember to document dates, times, names, titles of everyone you work with salesmen, managers, and even maintenance workers. This will build your complaint making it stronger and more direct, I dealt with a douche from Montclair Infiniti, Shawn Tahari (Finance Manager), I bet the complaint he got the next day from BBB and Infiniti Corp rattled his cage for handling a customer with his poor people skills. Let's just say the next day when I went in he didn't even dare look at me, had has head down the entire time I was there.