The plans are out there. http://www.snowathome.com/free_plans.php
I am guessing the 500$ figure does not inclue the air compressor or pressure washer. I believe most of those plans were from an old snowmakers at home list. There is someone here on the board that used to build a snowpark in his back yard and had links to the forum in his blog. Can't remember the name. Perhaps he will see the thread and post.