It depends on where you live, what type of equipment you buy, and whether you DIY or hire an installer. How is this existing pond constructed? Is it natural, concrete, or rubber lined? Will make a big difference on the cost to retrofit. For a standard pond with bottom drain and waterfall, the filterfalls ($400-500), skimmer ($300-500), UV ($200-400), pumps ($250-500 ea, you need at least two), and piping will cost $1500-2000, and that's just for the equipment. For ducks you really need a bog to help process the bioload. I would also add a second bottom drain to dedicated filter if I built my pond again, which is at least another $500-600 depending on the type of filter you choose. You want to buy quality stuff as it will be running 24/7 and cheap components will break down. You will not want to skimp on the filters because ducks make a mess. Just go to one of the online pond supply stores to get an idea of the different equipment that's available. We have Savio components. Cost will also depend on the design and landscaping. I designed the pond and dug the hole myself, but hired a pond contractor to install the equipment. It is nice to just call someone when there is a problem and they have to come fix it. You will save money if you do everything yourself, though. There are tons of awesome videos on YouTube on how to build a koi pond...