None of the Signature restaurants at the resorts are open for lunch.
The only Sig currently in the park is the Brown Derby in DHS and it is 2 credits for lunch or dinner (same menu all day).
LeCellier in Epcot is scheduled to become a Sig on March 1. At that time it will be 2 credits for dinner, but it will remain 1 credit for lunch. Currently it is 1 credit for either.
The only restaurant that changes the number of credits is Le Cellier (1 for lunch, 2 for dinner). All the other signature restaurants are 2 credits for breakfast, lunch (if served) and dinner.