Just got quoted 4-5000 an acre..
But the most important thing is by who?
There have been horror stories of guys taking a low bid from a guy that claims: "yeah I've built a few ponds in my day".
Of course I am sure you will look at his other work and talk to his customers before deciding.
If you can get a 2 acre pond built by a professional for $8000 that is pretty sweet.