I think a cast receiver would probably require more finishing work. It wouldn't really save any money, just move the cost from the initial tooling setup to the final stages of production. What I'd like to see is more polymer lowers. Magpul needs to get on that
I don't think it would require any more machining than a forging. Have you seen a raw forged lower?
AR15 LOWER RECEIVER RAW FORGING - Tactical Innovations Inc. - Idaho
Trust me, you can do that with a casting. Easily.