How to keep costs down for a party?

go and rob a bottle shop, go to bakeries at night  and get the day old cakes etc from the bin.

Buy a live pig or goat & self butcher it. Or catch a large fish or buy one. 2 to 4 rice dish's. Buy vegetables to steam at the market & a good home made honey dip of wild honey. 

No to helium just blow up regular balloons.

Try getting guests to BYO drinks, but say food and some drinks will be provided.

Cheap eats include frozen pizza that you can add extra toppings and cheese very cheaply, and is a hot filler.

Food that can be prepared ahead of time is useful - cocktail sausages, cheese cubes, salad bowl, mini ham sandwiches, indian snacks work well too.

Nobody will be there to eat a feast so is just a fun time to talk with nibbles if people get hungry.

go and rob a bottle shop, go to bakeries at night  and get the day old cakes etc from the bin.

Skip the balloons 

Just decorate one table nice

Ask the guests to 'bring a plate' like a 'pot luck' event, then you only need to supply the 'basics' at the venue.

Taco bar? Yuck! Helium balloons?? No, very juvenile and the helium stores are rapidly being depleted by moronic balloons. Just NO on that

Skip the helium tank. Get a few balloon centerpieces and some big numbers for the anniversary year. I love the idea of a taco bar, how about some margaritas to go with?

It's bending etiquette, but many families will make parties like this a group effort especially when a 19 year old college student is bearing the brunt of it.  

Do you have siblings or aunts/uncles who might be willing to help?   I wouldn't ask anyone for money or help directly, but you can spread the word that if anyone would like to bring paper plates/napkins/cups or pick up some beverages or bring the guacamole, that would be great!