I am assuming you mean one of the prefab ponds.
Hmmm. Tough question. How big the pool, where will you buy it, what do you want to put in it? How about landscaping around it? Will you do it yourself or hire someone to install.
The pool/pond prices vary greatly by size and source. Sometimes you can get good deals at the end of the season. Pumps are expensive, $30.00 for a smaller one, but should last for years. You can make a small, attractive pool without a pump, but the sound of moving water, plus the extra oxygen/circulation, make them a good purchase. I advise goldfish over koi etc., esp. for a small pond. Keep it understock and underfed and your water will stay clear. Overstocking and overfeeding cause problems. I pick my goldfish out of the feeder fish tank in a pet shop, you can pick some pretty ones out of the multitudes.
I would advise getting as large of a pond/pool as you can afford and fits your space. The larger the water volume the better buffered the pond is agains mistakes, fluxuations in water temperature and chemistry, etc.

Get a Viking pool. It is perfect for freeze thaw climates like in NJ. You get a lifetime warranty with the pool, I got mine from Pool Designs Inc in Yardville NJ. They are great and easy to work with, i suggest you look into that. Good luck