Sometimes they are just harder to find, not many people use them as main feeders they didn't really grow on as much as waxworms I don't know for certain, But I only use them as a treat so it doesn't bother me payin £5 for a load.
They cannot be bred in captivity, as they are an invasive species. They are found somewhere in South America in a certain species of tree, and a sprayed with a chemical that makes them infertile. That means all the butterworms you buy have been imported in. Therefore you have to pay a larger price.
If you only want to butterworms for their high calcium, I’d suggest you go with phoenixworms.