cheese, 3-4 pounds. A 5 pound block sharp cheddar at Sam's costs about the same as 2-3 pounds anywhere else, and it's good.
Consider adding 1 pound cream cheese with a jar of good preserves over it- could be red currant, apricot, sour cherries, fig.
crackers, 3 pounds. If someone in the group is a bread baker, ask them to make you 4 pounds of great home-made bread- people love this and it is cheap

chips, 5 pounds. Have a quart of salsa and a quart of another dip.

a quart of tiny pickles, a quart of olives, maybe some grape or cherry tomatoes

If you can, add a simple fruit tray (see the fruit tray page) and a punch or fresh lemonade, made from frozen concentrate, about 4 gallons- throw in a can of limeade and a can of tropical or orange mango and float some sliced oranges, lemons and strawberries on it.
20 pounds of ice. 100 small plates. 100 smallish cups. 200 napkins. toothpicks