One of the easiest and least expensive is a tub or block of cream cheese, topped with either hot pepper jelly or chutney. It always disappears, but only takes seconds to make, and either put out crackers, slices of bread, or wedges of pita or some other kind of flat bread.
Never underestimate the power of potluck. In my social community, we're all 50-70 years old and all live on restricted budgets. (I'm close to the baby at 55). We're all happy to get an invitation and more than happy to bring a dish. I usually ask folk to bring "finger food" ... depending on the season it can be anything from a box of cookies to nachos to ... well, anything.
A taco bar can be fun and cheap. Refried beans, taco-meat, chili, chili con queso, etc set out with bags of chips and a stack of tortillias is always popular (with a variety of salsas, some shredded cheese and lettuce).
Hmmm .. now that I think of it I always have a slew of kiddies at my parties, so saying we're all middle aged or above must be a lie.