Aside from purchasing the meat to grill, one of the biggest barbeque expenses is the alcohol.
Serve beer and wine instead of hard alcohol to save money while still providing the buzz. You can get an inexpensive, yet still delightful jug of wine for as little as $6.99. And let guests know they're welcome to bring their own. Save even more by checking the local grocery store deals in their weekly mailers and shop the brand on sale.
For non-alcoholic beverages, provide a few different types of soda for your guests as well as flavored water you can make yourself. Instead of buying pricey individual sized sodas in cans or bottles, buy one or two liter bottles and keep them in an ice bucket to stay chilled during the barbeque. and then slicing limes, lemons, oranges, berries, etc. to infuse it with a citrus or fruity flavor. Guests will still have their soft drink of choice, and you will save a bundle by buying the liter bottles instead of cans.
And remember water is free! Jazz yours up by pouring it into a decorative dispenser and throwing in an umbrella!