It is true that in Malta there are grocery stores that focus exclusively on tourists, and,
consequently, the prices are high. But, there are also shops for locals where everything is 2-3 times cheaper. So, we recommend that you first find out where are all the "shopping" places of the locals.
"Many people go to large chain supermarkets like Lidl or Carrefour – there are good prices and standard sets of good quality products."
For understanding the living costs in Malta let's compare the two most popular touristic cities Valletta and St. Julians: