When it comes to valuation, there are quite a few aspects to look at.
1. First and foremost, quality
Quality is not always visible by looking at a koi, and more specifically by viewing a photo.
One of the main factors that influence the value of koi is the bloodline and the breeder.
First of all, if the koi is imported from Japan, you can expect from the koi to develop into a valuable and good looking fish. The Japanese are very proud of their fish and will not sell fish of low quality. Their reputation is extremely important to them and they are also under pressure from fellow breeders and societies in Japan not to sell low quality koi. They only keep the best and all low quality koi will be culled. In other regions, South Africa included, a breeder will often sell low quality koi because they often try to make as much money as possible to the detriment of their or their regions reputation. No other country in the world can match the success of the Japanese when it comes to breeding. They have established their breeding stock over decades and generations. A small koi bred by them may look just as good as a small koi from another country, but as the koi develops, the latter will show defects like shimis developing or colours starting to fade. One should not only look at the looks of a koi, but you have to consider the reputation of the breeder and from which bloodline do a koi come from. Two Kohaku's may look very much the same, but I will pay five times more for a fish that has been bred by a well known dealer as apposed to a locally bred koi.
2. Supply and demand
The normal principle of supply and demand influence the price of koi. A few years ago there were numerous earthquakes in the homeland of koi being Niigata, and that dramatically influenced the prices of imported koi because of the unavailability of koi. Some breeders in South Africa have flooded the market a while ago with cheap low quality koi. That influenced the value of koi. Cheap koi are currently being imported from Israel and Singapore. This is affecting the market.
3. Time of year
Different times of the year, the prices of koi are different depending on when most local breeders spawned, and how many koi importers imported koi recently. Most importers import koi during October and November (mainly two year old koi) or during March, April, May (Tosai). Soon after these imports, you get Japanese koi cheaper.
4. Private / from dealer
With buying koi, it is all about risk. You can often buy koi cheap from a private individual, but you are running huge risks. You don't know if the individual's fish was exposed to KHV. He might have bought a new fish in May which infected his whole collection, and KHV will only show during September when the temperature heats up again. The individual migh have encountered health problems, and therefore decided to sell his koi, and this happens very often. As soon as you add the newly purchased koi to your pond, you realize that these koi are infected with a bacteria and it often happens that buyers loose all the fish due to a "bug" introduced by the newly acquired koi. Dealers on the other hand quarantine their koi professionally, they will not sell fish unless they are 100% positive that the fish are healthy. To do this is coosting money, and they are running huge risks. A local importer imported R 120 000.00 worth of koi recently, and lost almost all due to health problems. Therefore, it is quite common for dealers, and more specifically importers, to add up to 500% to the price they are paying, and despite this, I believe that it is better to buy from a reputable dealer, even though they sometimes sell at 500% more than what the same fish might be from a private individual. The prices also differs dramatically from dealer to dealer. The better the reputation of the dealer, the higher the price.
5. Region
The region from where a dealer operates, influence the prices of koi. Dealers at remote areas often charge much more for koi, due to the lack of competition. The koi market is much bigger in Gauteng and Durban, and the prices are also much higher in these areas as apposed to the Cape Province and Free state.
6. Varieties
Certain varieties are much more expensive like the Gosanke group of Kohaku, Showa and Sanke. Rare sought after fish are also much more expensive like the Ki Utsuri and Kikokuryo.
7. Amount of fish that you buy
Normally, dealers would give you three fish for the price of two, etc. The more you spend, the lower the price per fish.
As you can see, there are so many factors that influence the value of your fish, and there are a huge difference between the normal value of fish and the replacement value. As a dealer, I would not have bought your fish for more than R 200 / fish (based on photos) If you have to replace your collection, you will probably pay R 600.00 to R 800 / fish from a dealer depending on where you are.
However, I can not assess your koi from photos. One of the very important qualities of koi are the skin quality. I have seen koi on photos where skin quality looks great, but when I see the fish itself, I notice that it has bad skin quality. The patterns can be noticed from a photo, but skin quality can not, and the latter is more important. Light settings on a camera also dramatically influence the appearance of koi. The hi (red) on a Kohaku often looks deep on a photo, but when you view the actual koi, you realise that the hi is actually fading and not deep at all. This alone can be the difference between R 100,00 for the koi or R 1000,00 for the same koi. One can take a photos and enhance the colours of the koi dramatically with any image software.
A kohaku of 20 cm with a nice pattern and bodyshape and no demerits can trade for R 2000.00. If it has one shimi (black spot) on the side that you could not view from a photo, it might only be worth only R 200.00.
It appears from your photos that you have a few nice pond fish but nothing exceptional. It does not look like imported Japanese fish but I may be wrong. It would have help tremendously if we knew where the fish comes from.
My suggestion is to request a local dealer to give you a written formal quote to replace your 40 koi of more or less the same quality. At R 600 / koi, you will be able to get a fairly decent collection. If you wish to replace the collection with fairly good Japanese koi, you are looking at minimum R 1000.00 / koi.
Any other views (from other forum members) will be appreciated.