This is what was reported for the SE versions:

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It ultimately cost $15 million to restore all three films. Fox, which owns "Star Wars," put up $10 million to refurbish the film. Lucas, who owns the subsequent films, contributed $5 million to revamp "The Empire Strikes Back," which arrives Feb. 21, and "Return of the Jedi," due out March 7. (Fox will spend in excess of $15 million to launch the re-release.)

http://articles.latimes.com/1997-01-28/entertainment/ca-22867_1_star-wars

Disney would be fortunate to be able to come up with any money near that amount to restore the originals. Although, there are a lot more variables to contend with now, than there were in the past. So taking into account inflation and other variables, it would definitely be higher than that.

Team Negative1