The Prequels CGI was not cheap or quickly. That is a disservice to what was done there. It set a brand new standard for what could be done.
This^ People seem to think this stuff just jumped out of a vacuum, when instead it was an evolution. Lucas has always pushed the tech envelope, and his choice to use that much CGI was because it was suddenly available to do. Prior to that, nobody had achieved anything like it, not to that extent. It was a risky decision, and ultimately poor because of a variety of reasons (lack of acting feedback, effects too clean, etc), but he couldn't have known that at the time.