Like all hobbies pond and fish keeping is expensive. Beyond the initial outlay of cash for the pond build and the purchase of fish, there are additional expenses one must consider.
Electricity required to run pumps, UV Sterilizers, Air Pumps, and Lights. Added an average $30/month for me.
Food – Premium koi food is expensive. If you can buy a large bag at the beginning of the season your cost per pound is lower. But if you live in very hot and humid area it may not be feasible to keep the food fresh. Add $25-60/month.
Water changes – Here in the US it is not too bad. I generally run from $12-$20 per month. It will be more if you have a larger pond or do daily filter flushes.
Medication – $50-100 per year assuming you only have minor issues to deal with. There are some basic items you will need to keep on hand like potassium permanganate, fluke meds, ich meds and anti-fungal meds.
Additives – Things like bacteria, algae killers, and chlorine removers can get quite expensive. Adds $20-$50 per month
Equipment items – I replace my filter pads annually which runs a little over $100 per year. Water Test Kits are relatively cheap and once you know how your pond behaves you can get by with just a couple of kits per year.
You will have additional related expenses such as books, magazine subscriptions, club dues, seminars, plants, travel expenses to see other koi/fish, etc. This hobby has all kinds of ways for you to part with your money but also is very personally rewarding. You are caring for living creatures and trying to provide them with the best environment to live in and thrive.