well at a young age they are insect feeders, so it all depends on what you feed them.
Crickets can be expensive if you have to buy them constantly, think its around $25 for 25 at a pet store. However you can start to breed your own and save big $$$ but if you should choose to do this i suggest you get a steady supply ready before you make the purchase of the dragon.
If breeding insects isnt for you then you can also feed him mealworms, cricket, locusts dusted in calcium/minerals as needed, this would be a stable diet until he can be moved onto vegies. I have heard of them eating cat food but would be weary about feeding such foods. I believe in keeping it as natural as possible, crickets are not natural but they are a form of live food and provide simulation.
WARNING: if feeding crickets donot leave them over night in the tank, they have a habbit of annoying your beardie and will nip and bite at the toes and eyes!