I love the deli roast beef idea. But you said you wanted things inexpensive. To make my idea work, you have to have a rotary slicer that can shave the meat from a roast.
Purchase either and inside round or sirloin tri-tip roast. Figure about a quarter pound per person (for 100 that's a toatal of 25 pounds of meat). Fire up a charcoal, kettle grill. Dived the coals into two piles on either side, leaving a 6 inch space between them. Light 'em up. season the roast with salt, pepper, and garlic. Place a throw-away aluminum laof pan, filled half way with water, between the charcoal piles. Add chunks of apple or maple to the fire. Place the cooking grid over the coals.
Put the roast over the drip pan. Cover the barbecue grill and close all vents half way. Roast without touching for about ten minutes per pound. Test how done it is by placing an instant read thermometer into the roast center. When it reads 135 degrees F. Remove the roast, place in a plastic bag and throw in the fridge. Let sit overnight.
Take the roast out in time to shave it into sandwich sized pieces. Use the roast beef to make the sandwiches.
This isn't as easy as bying deli meat, but it's a whole bunch cheaper, and in my opinion, it tastes much better. I did just this trick at my oldest daughter's wedding last year. It was a real hit.
Another great, and even cheaper way to do this is to barbecue a turkey, using the same method described for the roast beef. But don't take the turkey out until the thickest part of the breast reads 155 degrees. No need to baste, though brining would add flavor. Just carve the meat into thin slices by removing the breast muscles as one big piec, then slicing very thinly accross the grain. Of course you can shred the dark meat and use it as well, either for turkey salad sandwiches, or as is.
And you can do the same thing with good pork roast, or a fresh ham.
For something a little different, tnederize and grill some chuck or round steak. Dice and place in a pressure cooker with diced onion and diced green peppers, cilantro, garlic, salt and pepper, and a bit of your favorite hot sauce. Cook for a half hour and serve with diced tomato, guacamole, shredded romaine lettuce, and sour cream, all to be used with flour tortillas. They are carne-asada buritoes, but with tender carne asada meat because of the pressure cooking. Also have plenty of shredded medium cheddar cheese with this, and some good salsa. Oh, and don't tell people that you're serving burritoes. Tell them that you're giving them wraps. It seems that the term is "in style" right now. And if you purchase small flour tortillas, it makes the fillings go a long way.
Seeeeeeya; Goodweed of the North