I have a feeling you already changed them by now, but just for info for anyone else interested, my sentra is my learning car. Not my first car, but the first one that I've been doing my own maintenance on. Knowing that, I did the spark plugs about 3 weeks ago and it was very simple.
So if you're leery about it, read up on it, watch videos (doesn't necessarily have to be sentra specific) and go for it. It took about 20 minutes and cost less than half of what the OP was quoted.