Get a few quotes, but understand it costs ~$1K a day to run those machines for 10 hours. There are a lot of variables going into digging a hole, and especially one that will hold water well and not be an eyesore.

If soils are difficult, it may cost more. If they hit some great groundwater flow, you may need to pump and it will cost more, you may hit a vein of gravel or sand and then you may need to bring in clay, etc. Just plan on your quotes being +/-40%, but make sure and hire a contractor with a good pond building record. Also get the book on this site and read through it "Perfect Pond, Want One". I wish I has this resource before I started, but if so, I may have been scared off before I had begun.

Once you choose a contractor, have them come and verify the site has good soils with some test holes over the area you are planning on having the pond. The test holes will provide the pond builder with a great idea if the site has good materials for building, and if the site may be too wet to dig or not.

I have a little over 1/2 acre pond, and it cost me $9K, and averages about 10' deep. It is on a hill-side so it needed enough materials to build the dam. My initial quote was $6K, but the cost went up when my clay layer turned out to be closer to soft concrete. The big dozer struggled to dig it out.

It was all worth it in the end.

As a side-note: If using it for swimming, a 1/2 acre pond should be more like 6-10 feet deep or more. You don't want to touch bottom when the water is low in the middle of summer, or have weeds trailing up between your legs. You may fluctuate a few feet depending on available water, so aim for deeper. Having a shallow pond for safety reasons doesn't help as the bottoms are typically not stand-worthy, being slimy and muck-covered.

Also at least in NY, 9K is competitive with a good pool when you factor in fencing, decks, and supporting equipment.

Do the pond right the first time, it if very VERY expensive to go back and fix mistakes. Great site to get started on though!