To start with, decide where you want to have the party. You can save some money by having it at your home, on your rooftop or at a friend's. That will allow you to buy your own liquor instead of going to a venue that charges a lot for a bottle of wine. You can set up coolers of beer and sodas and let folks serve themselves wine. You could also opt to hire a bartender and have him or her mix drinks with alcohol you buy at Costco or your grocery store.
If you can't afford a full-course meal, you can go with some kind of theme and make easy foods or apps that go with it. Mexican food and margaritas are easy to make, inexpensive and festive (including the fun decorations!). You could even stoke up the grill and let people assemble their own shish-ka-bobs, or order from a local BBQ joint that will bring a mess of meat, biscuits, beans and other sides for a decent price. They'll probably throw in all of the wet naps.
If your friends are a little more high-brow and you want to go as elegant as you can, serve only appetizers and cocktails. You can have your kid sister wear black slacks and a tux shirt and pass them around. Have a champagne toast. Perhaps you could have a beautiful dessert tray and offer coffee, teas and wines. Stoke up the espresso machine. Have a wine party and serve cheese and breads and olives. Just make sure there's enough food to soak up the booze.