A few previous posters listed prices so here is how Aulani prices compare to the rest of the Oahu:
Makahiki prices and quality are in line with what you'll find at similar buffets on the island. In a nutshell, it's overpriced for the quality unless you're a big, big eater.
Breakfast at AmaAma is a good value. We ate there last mother's day and paid less than $200 including tax, tip, extra sides, and extra entrees for a party of 4 adults and 2 kids.
Our family consisted of 2 adults and one very hungry 2yo during our last Aulani stay and we consistently spent $40-$50 on quick service meals, including drinks and desserts. The quality was just ok. Except the coconut shrimp, which is hands-down the best I've ever had.
Restaurants in Kapolei are priced for locals, and with the exception of a few locations, do not target the tourist market. A take-out meal at Zippy's (a local institution) will cost you about $10-$12/person and includes an entree like Korean chicken (my fav), two scoops of white rice, and one scoop macaroni salad. Zippy's has a table service area with slightly higher prices. The food court at Ka Makana Alii Mall has about a dozen eateries you won't find on the mainland.
If you go to bitesquad.com and type in Kapolei, the site will generate a list of restaurants and you can browse their menus/prices. Some restaurants charge a tiny bit more on bitesquad, but you can get a really good idea of what local restaurant prices are like.
As far as groceries, check Instacart.com. The online prices for food are consistently 20% more than what you'd pay in store. The markup on alcohol (which Costco and Safeway are not delivering at the moment) can vary wildly.
If you need to throw a budget meal into your itinerary, I HIGHLY suggest 7-11. I know how strange this sounds lol. 7-11 stores in Hawaii are Japanese owned and the sushi is very, very good for the price. You can also get ham and cheese breakfast sandwiches made with Portuguese sweet bread, manapua, pork hash, Japanese canned coffee, Nanding bakery bread fingers, spam musubis, garlic chicken musubis, and all kinds of local favorites. It's not haute cuisine but it's excellent quality for the price.