According to an expert (my wife), on average for our weekly shopping, for big supermarket Lulu is the most expensive. Giant is slightly more expensive than Carrefour, but she prefer to shop there as it is less crowded and parking in Hyatt Plaza is easier than villagio.

The cheapest and most complete in term of ingredient for southeast asian food is actually FFC near the airport. However, during the evening it tend to get very crowded and you end up spending a lot of time at the checkout counter.

May not be statistically correct though

Cheers
SK