Where to buy groceries

I have been taking the opportunity to shop our groceries in different stores to be able to determine the lowest prices. My general observation is that it seems these grocery stores are talking with each other. There are items which are cheaper and expensive in one store, but vice versa in another store. There are items which can only be found in certain stores. So where do we do our groceries..  it's the store near us with free parking. The following are our grocery store preferences:

1. Pioneer Center, Kapitolyo
     > generally cheaper for all grocery items ie oil, canned goods, distilled water, formula milk
     > free parking
     > exclusive item bought/usually available - drypers, ram beans
2. Robinson's Supermart
     > a bit pricy compared to Pioneer Center
     > free parking for receipts with more than 1,000Pesos
     > exclusive item bought/usually available - bundling of items, free taste
3. SM Store
     > generally same price with Robinson's ie cheapest tag price for the formula milk we are buying
     > no free parking, we only do our groceries if we need to go to the SM Malls

I have been also to Pure Gold and Cherry Fooderama which are also generally same price with Pioneer Centre. I will still have to do my groceries in Super8 which I heard maybe the cheapest.