If done right, $48,000.00 (at least!!)
for $20,000 you could maybe get the supplies, but very little labor.
After proper filtration for koi, pumps for streams and falls, lighting, mulch, rock, liner, lumber for decking, and cheap labor $48,000.
I did a 23' x 16' with a average depth of 66", Upper pond 10' x 5', 40' stream split into two cascading fall, with no lights but some extra patio and landscape work, my cost for materials was over $20,000. Pumps alone were $1,000. I hope nobody here is installing something like this for $6,000. Building ponds is fun, but paying your customer to let you build them one, takes all the fun out of it.