Nitro is one of my favorite machines as far as backglass art. I have bought and sold several of them over the past 3 years or so to end up with a super nice one and here in the North East that price seems about "dead on" for that condition. If backglass was nice with minor playfield wear it might be closer to an $800 machine but with backglass less than perfect resale after it is shopped is about $700 or about $600 before shopping it.
On a good day you might get as much as $850 for that machine but $650 is more in line in that condition. So if you are buying it to enjoy you are fine but if you are buying it to flip there isn't going to be a lot of profit in it.
And lastly if you are buying it for the "art" than it is a great machine but if you are buying it to play there are plenty of better titles you can buy cheaper. Personally I am torn - I currently own a super nice one but I am having a hard time keeping it in the collection because it doesn't get played. My wife keeps telling me to buy and hang a backglass if I love the art so much but sell the "stupid machine" because no one wants to play it.