It's all about cost and convenience. The only way you'll ever prevent movie piracy is to offer a legal alternative that is as easy and convenient, and priced in a way that makes the legal way the easy choice.
Charge a dollar or two for a regular download, double it for HD and new releases. I doubt most people would even bother to get a copy from a friend because it would be so cheap just to get it themselves.
Some people are never going to pay, some people are always going to pay. You're trying to catch the people who currently aren't paying because there isn't a sensibly priced legal alternative that's as easy as just firing up utorrent and the pirate bay.
Surely small amounts from lots of people is better than a slightly larger amount from a few people?