A useful site for answering these questions is operated by the U.S. Department of Agriculture (USD): http://www.cnpp.usda.gov/USDAFoodPlansCostofFood.htm

USDA measures the monthly grocery basket cost for children, men, women and families of different ages, based on four levels of spending: thrifty, low-cost, moderate-cost and liberal. The data excludes the cost of food eaten at restaurants. At this site, you can see a database of average costs going back 20 years. It offers perhaps the best overview of how U.S. grocery cost inflation has gone over time.

For a couple in the age 51-70 range, the average monthly cost of food at home currently is $588 under the moderate-cost plan and $710.30 under the liberal plan. For a family of four with one child age 6-8 and another age 9-11, these costs rise to $1,054.90 and $1,283.10, respectively.

Going back 20 years, this database shows a consistent long-term trend – namely, the cost of food at home has relentlessly increased by about 2.8% to 3% per year. Over the first five months of 2014, however, grocery inflation has increased to the 5-6% annual range. Whether or not this signals a "new normal" remains to be seen.

You can use the database to help your clients see if their grocery budgets are in line with the USDA norms established for nutritious daily diets, for different levels of income. Also, you can use "food at home" to keep clients mindful of the long-term impact of inflation. Even at 3% inflation, the cost of groceries doubles every 24 years.