Cheap Prepared Food Ideas for a Large Group

It all depends. In general, preparing from scratch is less costly and healthier, depending on the ingredients. Plus it's easier to cook in bulk and freeze additional portions for other meals.

For that large a group I think you need to get a caterer. They need to keep the food hot and cold. If you do it, it will take a lot of man power to keep the temps. Shop it around to several places and let them know you are shopping around. You should get the best prices if they know they have to cut to the bone to get the business. And in these times that is good.