Little St. Simons Island looks so wonderful ... and is so very much out of my price range. Does anyone know if there is a similar resort in the Ga./SC/NC coastal area that's not quite as expensive?

Try Jekyll Island which is right next door and has some less expensive choices for accomodation.

Almost everything is less expensive than the Lodge at Little St. Simons. Directly south is Sea Island, a world class destination. Try the Cloister. On St. Simons, you might investigate Sea Palms (in the marsh) or The King & Prince (on the [sliver of a] beach). Or, The Lodge [at Sea Island], the 5 star branch of the Cloister situated on St. Simons.
There are also accommodations on Cumberland Island and St. Mary's, where JFK, Jr. was married, that you might inquire about.
While Jekyll is close, it is not next door to Little St. Simons. Sea Island and St. Simons are directly south. Jekyll is south of St. Simons and a decent drive or challenging swim. I like Jekyll, especially the dunes and lack of development, though the accommodations and dining choices are mediocre.

If you don't mind going further south the next barrier island (after Cumberland) is Amelia Island. Slightly different flavor, but still southern with beaches, history and a nice downtown area with restaurants and shops.

If you are staying a week or more, it might make it more affordable to rent a cottage or condo. There are tons of realtors that handle rentals. I've rented twice on SSI itself and loved it so much, I'm planning to move there within the year!

I usually go to google and put in the following- saint simons island + cottage rentals (or condo rentals) There is also a website vacation rentals by owner http://vrbo.com/vacation-rentals/usa that always has some interesting looking places.