This will depend on the size and depth of the pond, if you need to buy materials like rocks to line the pond, and the cost of any landscaping and plants both for the pond and surrounding it. You could build an inground 4x6-foot pond using a pond form and a submersible pump for a few hundred dollars. You can do it even cheaper if you're willing to search for a used pond form at yard sales or in online classified ads. An 8x10-foot inground pond, made with a pond liner kit, complete with a waterfall and 2000 gph (gallons per hour) pump, lined with rocks from a landscaping business, and stocked with aquatic plants and goldfish could cost between $2,000 and $5,000 US. A larger in-ground pond installed by professionals could cost $10 to $20,000 US or more. I've seen magnificent 20x50-foot garden ponds that cost $100k or more. I've also seen others that size and budget that were ...not good! That's why it's good to have an idea of what you want first, and, if hiring help, be certain the builders share your vision and your budget will cover it. You could also build a little patio container pond in an old barrel with a liner and small fountain pump for $50 US.