Restaurant supply stores usually are cheaper, but they do not care to sell small amounts. You might want to buy your staples like flour, sugar, and some canned goods from a restaurant supply store and get enough for more than one event. Then buy the specialty items that you will not be using over and over again at a chain warehouse store, or even at a regular grocery store. Look for bargains in the ads. Prices you pay will depend upon quality of merchandise.
Fresh fruits cost more than can fruits, and the nicer fresh fruits cost even more.
Keep an eye on how much you are spending and make sure you don't overspend on insignificant parts of the menu. The most expensive part of the menu should be the entree. It will be what the diner will remember the most and will be what makes the meal great or ruins the meal. Check your recipes, add the amounts together. Look at the chart on this site that tells you how much to buy for 24 people.