Thanks. On Savaii chances are you will be eating where you are staying, unless you have a rental car, or a resort is in walking distance. If you book an open fale on the beach, the price usually includes breakfast and dinner so most meals will be included in the rate. If you stay at a resort, breakfast is usually included and you pay extra for lunch and dinner, although some may have meal plans you can pre-purchase so you know when you book how much you will be paying for food.
There are no 5 star resorts on Savaii so food is not going to be really expensive. Have a look at the Facebook pages and websites of the accommodation options you're considering and you should get an indication of food and prices, if not, drop them an email and ask if they could send you a menu with prices.
Enjoy your holiday planning, it's exciting to look at all the different options and to know you will be there soon enough to experience it for yourself. Happy travels.