we use our bbq pretty much every night during the summer and what I've found is we have to remind ourselves not to cook more than we normally would if eating the identical meat cooked another way. it seems like all that open grill space just screams for throwing on way more burgers or chicken or ribs than we normally would plan for a single meal. to address this we continue our non bbq season habit of purchasing meat at Costco or when it's on a good sale and breaking it into single meal size packages for our household. i know how many pieces of chicken or how many ribs each person should have for a meal so we batch the packages accordingly and use our air sealer to put them up in the freezer. if we were cooking burgers in the house no one would normally eat 3 and then also have hot dogs so we scale back and just do the number of burgers or dogs that are appropriate (or if we are doing up enough for dinner then lunch the next day, lunch's portion is put away before we eat dinner to keep us from snacking lunch away).
as for snacks and sides and such-bbq beans made from scratch are much less expensive than premade, same for coleslaw and other salads. we tend to go with whatever veggies are on sale-corn on the cob gets very reasonable around here so it's a fairly regular item, but whatever is on sale can get seasoned and cooked in a foil pouch on the grill. fruit prices get wonky here but watermelon is often decently priced and if i pre-slice and just put out an appropriate amount it doesn't get inhaled in one sitting. i personally think multi layer dip is one of the most inexpensive items to make-i always have canned refried beans on hand that i can top with a layers of salsa, grated cheese and sliced olives or ortegas (my kids aren't keen on sour cream or guacamole which is fine b/c it saves money). i get the large bags of tortilla chips at
walmart for this or just to have with salsa (which i stock up on when it's on sale). when my kids were younger they loved those awful otter pops so i would keep the freezer stocked with them. apple crisp is one of the most inexpensive and easy desserts for me to make so it's usually on hand.
meat savings? i basically just watch, watch, watch the ads-seems like around memorial day there's always some good ones on hot dogs, all types of buns and condiments so i stock up (and freeze what i can). country boy ribs go on sale seems more often in the winter and spring here so i stock up ahead of time. chicken value packs go on sale frequently so i end up w/half meal sized packages so i can make it a mix of drumsticks and breasts (which we often butterfly b/c they can serve 2 people).
i would say to look at what you currently cook and figure out how to transform the proteins into bbq foods (the stew meat i get at Costco is plenty tender for kabobs, the pork chops grill up great....).