Following are some ideas to save money when planning a graduation party, or any type of party.
Check out the dollar stores. Some dollar stores have Mylar helium balloon that may be less expensive than other stores.
Also, they have plastic plates, napkins, utensils in various colors and themes. You may also be able to find serving bowls and platters. Also look for party decorations and items that could be used as centerpieces.
If you are in need of serving bowls, platters, etc another place to check out are the second hand stores. Some have various items like this at a fraction of buying new.
I was recently at Heirlooms at 3120 25th St S, Fargo and they had a few tables of serving items, such as silver and glass bowls and trays. Heirlooms is an upscale second hand store where the proceeds benefit the Hospice of the Red River Valley.