Well first off, Shedd is one of the oldest and most highly ranked aquariums in North America. Secondly, it is a reasonable sized facility, especially considering virtually all of its exhibitry is indoors. In addition, its cetacean program is going as strong as ever and there are absolutely no plans to phase it out or down size it. They are currently the only facility in NA breeding whited sided dolphins (3 calves since 2012), and have a very genetically strong beluga group with multiple proven breeders.
And of course admission needs to be less for local residents. Many facilities offer this as a method to encourage residents to visit multiple times a year, especially if it is a location that does not necessarily offer new exhibits on frequent basis.
30 dollars is very reasonable, especially when you consider everything the aquarium has on display. Or alternately, you can look at it like this. That's the same price as three baconator combo meals at Wendy's, or if you really want to compare, at the current time of this post (April 7th) the Shedds admission is 22 dollars cheaper than San Diego (30 dollars vs 52). Now of course, I'm going to have to assume your under 12 years old, as 30 dollars is the children's admission price of shedd, so retrospectively 30 dollars probably is a lot for you. In that case, save all your allowance and tell you mom to give you money for a shedd visit instead of getting you a snack from a fast food place.