I have a one acre pond, about the same depth, that is scheduled to begin being cleaned out next week. Right now I plan to have him work 40 hours with his big excavator at $175 and hour. I doubt that he'll get the entire acre dug out in that time. I'll probably add new info to this thread.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Main=22609&Number=286702#Post286702