I spent a lot of time looking into this. I finally got someone to send me the ingredients printed on their commercial silkworm chow and it was mostly made up of dry powdered mulberry leaves with standard starchy fillers to stretch it out. It's basically the only viable alternative if you don't have a reliable source of mulberry leaves. That's unfortunately the only drawback to silkworms in my book, they are a great feeder.
The cheapest way to do it is to buy dry chow in bulk and cook it yourself.