phoenix worms are a bit expensive, though they don't need to be dusted with calcium to achieve the proper Ca ratio (~1.2:1) like crickets and the rest of the common feder insects. Most of the large breeders feed single-item diets to their animals with no apparent problem at all. if you do use them as a staple diet, make sure to dust them with a good vitamin supplement.
It's true they can get expensive as a staple, they are a very small feeder as well. There usually in addition to a staple, it's more practical that way, but they are great because of there c/p balance. Do you have a local store that sells them?.