Well, I'm no expert in the subject so I just direct people to this old post from savestarwars.com from when George was still in charge.

"Lucas speaks about how expensive it would be and all the work that would need to be done, but in fact it would be relatively easy and inexpensive. One, presenting existing prints in high-def with modest cleanup would cost in the hundred thousand dollar range. Relatively speaking, this is very inexpensive, which is why obscure films like Police Academy 5 are available from original 35mm elements. Restoring the films from the negatives is not too expensive as well--because it was largely already done. In 1995, Twentieth Century Fox spent $20 million restoring and enhancing the Star Wars trilogy for the anniversary re-release. All that would need to be done today is retrieving the missing original pieces (roughly five to ten minutes per film), cleaning them if necessary, and editing them into a scan of the existing negative. Further cleanup could be done, but this is not strictly necessary, as the 1997 release had no major digital cleanup. So, the pricetag to finish the restoration of the trilogy from the negatives would be in the range of a million dollars, to throw a ballpark figure out there. To put this in perspective, the 2004 DVD set sold $100 million in its day of release. To put it in greater perspective, Lucas is a billionaire as it is. Lucas also need not involve himself in this, as film restorationists could handle the project themselves and seek to match the new digital copy to archival material. The project could be outsourced as well, to Criterion for example."