I just set a 18ft water fall with a 11X16 Pond 2.5ft deep, and the cost for just for the supplies was $6900.00, $3000.00 was just in Stone. Total cost to customer $14,000
So If you did a pond that size and the supplies only cost less than $5000.00. You had to use cheap stuff and not much stone. I hope you did not give a warrenty.
For your answear I would charge $24,000 for a project like the one you installed