FotR might break the billion dollar mark alone before it is finished in the theators. That is NOT counting the other two movies, the video/DVD release of the three movies and merchandise from the three movies.
Trust me. Just give it time. They will find a way to talk the Tolken family into let them make the other books into movies. A couple of hundred million dollars, a percent of the gross and/or profit of the movies might do the trick.
This movie has legs, with little competition over the next few months, word of mouth buzz and awards boosting it.