HaLeVia here.
KRM in Boro Park has excellent prices. Many large families do a lot of Costco shopping.
The local groceries get plenty of use as well because of convenience and great service, so I don’t feel one bit guilty about pursuing good deals in bigger stores.
A good resource you might not happen to hear about is Rosner’s in either BP or Kensington. They use various cost-cutting means to keep their prices very low.
To save even more, you might want to consider alternatives to disposable goods. I use almost no paper towels (rags are free and can be reused indefinitely) and very few disposable cups. Cups are easy to rinse, and Ikea’s rainbow set of kiddie cups are excellent. We’ve designated colors for milchigs and fleishigs. For ten dollars you get two dozen cups and a free one-way ferry ride. (So consider saving that Ikea trip for chol hamoed or someone’s birthday.)