2004 Maxima needs fuel injector, sensor?

Hello, and thank you for having such a forum. I am a single woman with issues with my Maxima. It is a 2004 Nissan Maxima SE with approximately 39K miles and is a V6. At approximately 37,000 miles I had the recommended 30K service performed by Nissan. I noticed the car seemed to ride and idle rough and was slightly jumpy after this service. I took the car back the next morning and they stated they did nothing with the 30K service that would effect the performance of the car, but the technition rode along with me. The car reproduced the "jumpiness" on the ride, and the service tech told me that was normal and a lot of cars are actually worse than that. I swore to him that the car had not been that rough, but he stated it was normal. That afternoon, my Service Engine Soon light came on. I called Nissan and let them know. They said to bring it in. I was out of town and couldn't get the car back in until 39K, car continued to run rough when driving with the SES light still on, worse when I am at a stop and take my foot off the brake and let the car creep forward, this is when the car is at its' roughest, but it is rough when accelerating or maintaing speed as well. Nissan stated I need a new fuel injector (number 4) and air/fuel sensor. Along with the diagnostics performed, the total will be $771.79. Does this seem excessive for a car that is just out of warrenty? I have had all previous recommended service and oil changes and maintain documentation of this. I have always previously owned Honda's, and have never had a problem until they get near 100K, and I feel it is pretty reasonable at that point. This is extremely frustrating, and being as I don't know much about what goes on under the hood, I don't know if this dealership is trying to take advantage of a single, blonde woman. Any information or insight would be greatly appreciated.