How would that affect the Egyptian economy? Keeping half or all.
Probably not all that much in Egypt itself, but it would result in a bigger joint economy and give Egypt/Sudan a lot more pull in African affairs.
It would probably have a pretty big positive effect on the Sudanese economy, who would have access to a lot more trained officials and technicians, and as part of a larger polity it's possible better order would be maintained.
Eventually the oil would help, but I'm guessing the cost of extraction there would never have made it the most competitive, although earlier development might have taken advantage of all the various Mid East crises, and at least made Egypt-Sudan self-reliant.
I think it's in the political sphere this might have made a large difference. A massive Egypt-Sudan looks a lot more like the central focus of a unity movement, and if oil-independent is going to have more freedom of maneuver in diplomacy.