For an alternative to macaroni salad, try Israeli couscous (pearled couscous) which is larger pieces than regular couscous. You can find it in the natural food section, with the other rice and orzo products, or in the kosher food section (sometimes in a bag). Bob's Red Mill also makes it. Add in whatever veggies you have (traditionally it's cucumber, tomato, parsley but you can use celery, carrots, colored peppers etc.). Then toss with olive or canola oil and vinegar (I like rice wine vinegar) - no mayo. It keeps well in the heat and you can make it ahead.
Rice salad - make brown rice, add in various beans (black, red, pinto, white, or chickpeas), a bunch of frozen peas (don't cook, just defrost) and a light oil/vinegar mix. Make ahead, keeps well.
Hot baked beans - you can google easy recipes but here's one: http://www.food.com/recipe/crock-pot-baked-beans-from-scr...
Do them in the crockpot and keep them hot that way. This recipe has you using dried beans (cheaper than canned, which are still pretty cheap if you check the sales) and you start it the day before.
Beans, rice and couscous are filling and generally inexpensive.
If you have the grill space, slice up a bunch of sweet potatoes, Russets, and red potatoes into wedges (the reds might have to be in halves or quarters). You can add wedges of onion if you like. Mix up some canola or olive oil and some herbs (I like dill, salt & pepper), toss to coat, and grill. If the wedges are big, put them right on the grill. If the potatoes are small, cut in chunks and thread onto a skewer or use a grill basket (same with chunks of onion). They usually grill up in 20 minutes (less for the onions), and then you can keep them warm on the top of the grill, in a low oven, or in one of those foil pans set over Sterno. These are cheap veggies and they are filling. Sometimes asparagus is on sale and you can grill that too, same marinade. Put them sideways carefully so they don't fall through the grill though, or use a grill basket.
Raw veggies and humus keep well in the heat. You can choose veggies based on price. Celery and carrots and often green beans are economical, peppers can get pricey so I'd use those for "accent" color in the salads.