Here's an estimate of what is needed for a medium-sized dance party in
Hacienda Commons or the Oak room.  It would cover around 300 people.

Note: we probably have large quantities of starred items in the
Hacienda/Buttery area.

3 bags clear plastic cups (128/bag)
napkins*
plastic bowls and trays*
20 bottles of sparkling cider*
50 liters of wine, mix of red & white
7 cases of beer (look for Weinhard's)
7 cases of water
4 pallets canned apple juice
8 pallets various sodas
gummi bears
pretzels (5-lb bag)
chex mix (4 or 5-lb bag)
2 large bags M&M's
6 boxes various cookies
3 bags of chips
Miscellaneous candies: mints, jelly beans, tootsie rolls, skittles,
chocolate-covered raisins
200 lbs of ice (can be obtained from chemistry dept. ice machines if you
know a chemist & can carry the coolers/buckets there & back)

Spending for this party would be around $600, plus or minus a hundred or
two.

People tend to be offended by boxed wine or generic soda, so these should
generally be avoided, even though they are cheap.

Costco is very cheap & is the preferred place to buy stuff but it requires
someone with a Costco card & a large vehicle.  Costco accepts only
American Express and Discover cards (cash & check too).

Beverages & More will deliver kegs, non-beer beverages, and snacks.
Beltramo's is another option.  These are more expensive but good for large
events or when the labor supply is low.

It's nice to buy non-perishable foods & drinks because leftovers can be
used at later events & you don't have to worry if you buy too much.
However, having fruit, meat, cheese, etc. can make an event more classy.

Diddam's is a good place to get decorations, props, and prizes.  It may be
possible to get restaurants, bookstores, etc. to donate gift certificates
if they are acknowledged in advertisements.