1) Booze - BYOB, but I did always have the odd person who would run out, so I'd have extra on hand.
2) Drinks for in-between, DDs - iced tea, hot tea - both super economical. Look in the freezer aisle for frozen juices from concentrate if you have to - dress them up with some fruit. A go-to for me is to have a nice big jug/decanter full of water with lemon slices. It looks super pretty and is thoughtful, without being expensive, at all. You may be able to borrow something like this from a family member or friend. Combined with tea or iced tea - you have your bases covered.
FOOD - If you have to serve food, turn it into a potluck and ask each of your guests to bring in a little something of their own. Potlucks are the best! There's always a lot of food, nobody feels like a moocher, and the cooking snobs get to show off. Those who can't cook can always pick up cookies or a pie from the grocery store. Also people with dietary restrictions can bring what they'll eat.
SNACKS - Then, I would pick up a variety of bargain chips. No Name chips work just fine, especially if you put them in a bowl, no one will know the difference. The Dollar Tree often carries party mix as well - the trick is having a variety (depending on the size of the party, I'd go with 5 bags of different munchies, and could usually achieve this for $4-$5).
CAKE - definitely go a boxed cake. I'm not sure if you have icing sugar at home, but you could compare pricing to see what is more economical where you live - a home made frosting here or a store bought can. You can also make a cake with a box of cake mix and a can of soda! It's actually pretty good! I do this all the time. Match the color of the cake to the soda that means sprite/7up/cream soda in a white or yellow cake, cherry in red velvet, cola in chocolate.
DECORATIONS - Do you actually need a banner that says "happy birthday?" Everybody knows it's your birthday. Better to go classy. Pick a color/theme and stick to whichever you choose: you could go 'picnic' with red and white checkered tablecloths from the Dollar Store, white and craft paper with an accent color is pretty, 'vintage tea party' is very nice-looking - tea cups with your treats in it, doilies from the Dollar Store will add to the effect. Look around in your environment and see what you can find. Wildflowers look awesome in mason jars as well as vases - they look great in buckets, even.
LASTLY - make a EXCEEDINGLY good playlist - music sets the entire tone for the night, so it's really what will make or break you!