Can you get any of the other guests to bring anything with them?
You could make big bowls of rice salad and new potato salad, and maybe get some ready to bake baguettes from the supermarkets. Tesco value tortilla chips were highly rated on a thread here a little while ago.
Chicken thighs, drumsticks and wings are quite cheap, you can knock up a marinade with garlic, soy sauce, mustard, honey and tomato ketchup. If you wanted to do something a bit different, mackerel isn't expensive and is great on the bbq - just get the fishmonger to clean and gut and stick some lime, chilli and coriander in the cavity before cooking.
Sausages are a must for a bbq. Corn on the cob is great too, as are baked spuds.
I wouldn't normally bother with anything sweet, but you could get marshmallows to toast on the dying embers once all the savoury stuff has been done, or slit bananas and stick a few choccie buttons in them and stick them on.
Unfortunately, nothing seems really 'cheap' at the moment - it would be great if some of your guests helped out.