Jmoney, I live in the Philadelphia area and I have to say that $700 for a spay for a big dog doesn't sound that unusual. About 6 years ago I had an 85 lb foster dog nuetered and most of the vets I called were quoting me $400 to $500. I had to really hunt (and really drive) to find a vet that charged under $250. Considering it has been 6 years, I wouldn't be surprised at all to get a quote of $700 for a spay.
And to answer the op, I really can't tell you what to do. It depends on the what you think of the vet at the shelter. They could be a wonderful vet, they could be not so wonderful. But the same holds true for any of the priivate vets you called. You really have to look into it on your own. So I guess my answer is that I would not eliminate the possibility of going to the shelter but I would have tomlook into it. I actually like the cheaper vet that I found for my foster dog better than any of the other vets I have found. He is very easy to talk to and very knowledgible. I now use him as my vet despite the long drive.