Lots of Side dishes?
They will fill people up.
How about:
Home made Coleslaw
Potato Salad (with chopped boiled eggs and some cheap offcuts of bacon through it,sweetcorn peppers, bit of mustard with the mayonnaise etc)
Feta (you can buy basics) and olive salad?
Where burgers etc are concerned, not sure if already made ones are cheaper, but I always use half good mince and half cheap as the fat in the latter gives more flavour.
Making skewers with chunks of meat - you can beef these up with bits of red onion, peppers and cherry tomatoes and mushroom to make the meat go further.
Cheaper sausages (not the really cheap though!) can be made to taste lovely if you slice them lengthways - make sure it's not all the way through - and spread with cranberry sauce and mustard then put them on skewers (to close the slice iykwim) and alternate with Bay leaves. Makes them taste v expensive.
Chicken wings are cheap and be glazed with a bbq glaze (ketchup, soy, honey etc) or chinese (five spice)for different flavour.
Sorry if this isn't what you were getting at in terms of help!