Cast isn't necessarily cheaper than billet. You first have to make a pattern and match plates, which runs about $4000. Then you need to cast at least 200 pieces to get a good price from the foundry. And finally, you still need to machine them. The drawback to cast, is that you can't make changes on the fly.
Billet cost less if you make less if you make limited quantities, and you can modify the programming to suit your needs, anytime you want.
Both look nice, I personally would prefer a cast cover to match my valve cover and air cleaner. However, I've been talking with my machinist and we may make several billet parts, including a valve cover, 10" air cleaner, timing cover, water pump pulley, t-stat, wire loom, etc. We may even do a kit like to ones March offers for V8s.