Well, what does a house cost? It’s the same kind of question with no concrete answer. You can look at the square footage of either but that does not dictate the cost. Even when you are comparing a 3 bedroom/2 Bath home to another 3 Bedroom/2 bath home, you will find variances based upon many factors such as location, access, the types of materials used, the upgrades, add ons, and all the special bells and whistles or lack thereof. One architect will get paid more for his design services, use the best materials and hire only highly skilled artisans when he builds his homes while the other guy may be cranking out houses left and right while using the cheapest materials and the cheapest laborers whose work is a bit sloppy. The same goes for water features as well.
Expect that each contractor will give you a different price based on his or her skill level and building style. Stacking stone is an art of form and function. Some will use smaller boulders and stone while the other guy may may use large boulders and bring in a machine. Just the difference in the tons of rock, and the labor to move this rock, will create a big difference in cost of your project. This goes for depth as well. Just a little deeper will increase the cost more than you’d think. Let's consider the materials. There are different methods of building and filtering water features, too many options to list here, but I am sure you could guess that the cost once again can have a wide swing.
Something to think about when choosing your builder is that the lowest bidder is not always the best choice. Having a pond or waterfeature done correctly from the beginning and paying a bit more for it, may save you thousands in the long run.
The best way to find out what a pond would cost in your area is to get a professional pond consultation and discuss your project needs and wishes with your local pond installers.