There's a lot of variables to building a pond that will affect cost, such as on site soil conditions, damming up existing drainage/stream or digging a hole in flat ground where cut has to be hauled off, amount of clearing required for pond, etc.

As far as downsides to building a pond, IMO there are basically none as long as it is built correctly. You will have to spend some time/money maintaining it but it provides another opportunity for you to enjoy your property. And it adds to the property value if you ever decide to sell.