I agree and that is why I buy inexpensive industrial food and supplement with other stuff.
We have a saltwater pond that becomes covered with Enteromorpha and Ulva (green seaweeds). The koi love it. A readily available source of seaweed are the sheets of nori sold for wrapping sushi. Depending on where you live, you may also be able to find dry wakame or kombu which is dry kelp used for making soup.
There are commercial sources of frozen marine bloodworms. These are large polychaete worms about 20-30 cm and are not the same as the little red midge fly larvae - although both are excellent nutrition. They used to sell the big marine bloodworms only for bait but now most of the production goes to shrimp hatcheries where they are used to help induce spawning in the breeder shrimp.
Maine Bait is a big supplier in the US and
Seabait is the major supplier in Europe. Last time I checked, the price was about $8 US/kg. That would be about $25 US/kg on a dry weight basis.
-steve