A good combo is feeding them a 50:50 mix of sinking catfish pellets and rabbit pellets. It's inexpensive and meets most ot\f their needs. After food, look to providing them iodine and calcium supplements. The 2 are needed to aid in the crayfish's molt and restore lost calcium.
I used to feed mine earthworms and sometimes lettuce. It also ate uneaten fish food that fell to the bottom of the tank...but yeah, they will pretty much eat anything.