Why billet? For small volume it's cheaper than a casting. Once you have the CNC program, the only cost is the alloy per lb., and the hours to run the CNC. The first and last one cost the same, and you don't need any inventory: as-needed for tomorrow only..
A casting has a huge set-up cost, the wooden model, then the mold, melting, skimming, pouring, heat treatment, manual finishing (yes, I took foundry and machine shop in school). The first one cost $10,000, the thousandth one costs $300.