I don't add it to my food budget at all. When I throw a party I make a separate budget for the party that starts with the food costs and then I add all the other party things to the budget from there.
I usually have a stocked pantry and 2nd pantry and freezer that I can pull items from. One dish isnt going to rock my budget.
For example if Im bring chicken casserole, I pull out a package of chicken $5, 2 boxes of rice a roni $2, bag of frozen veggies $1, broth $.69 misc spices etc... and for $8 i have a main dish to deliver....
If I have a kids b'day party its more snacks and drink boxes etc.. but its the same principle, we stock up and stockpile so everything we need is here.... for that we may MAY order pizzas to make it easy on me..
But, for covered dish or for taking a meal to a new baby? For those, I try to stick with frugal family favorites. I have a brunch covered dish once a month or so. I make muffins, do bread pudding, or make deviled eggs. I try to keep it under $2 or $3 for the dish (we take two dishes for each meal). I like to cook, so that helps. The people that don't like to cook bring the pricier pre-made fruit salad or drinks.
For new babies, I usually double what we're having for supper. I take things like sloppy joes, tortilla soup, chicken casserole, whatever. Like a pp said, it doesn't really break my budget. I rarely make expensive stuff for our family, so I just take the same things to my friends. They are my friends, they know me, and they don't expect shrimp and filet mignon.
If I'm bringing a single dish, I try to fit it into my budget - maybe by raiding the pantry/freezer, picking something that uses cheaper ingredients (lentils or pasta or beans instead of meat), or just squeezing to fit it. If I'm planning a party though, that's a budget all of it's own, usually.
You're really talking about two separate things here. If it's a dish to take to a shut-in or a family with a new baby or a dish to a potluck, that's easy enough to deal with. But if you're talking about a full-blown party where I'm the hostess and providing just about anything? If I don't have the $$$ and couldn't budget for it, I don't have the party. Period. Or you make it into a potluck and maybe provide drinks and desserts and ask folks to bring something. It's done quite often in my group of friends.
We entertain at home at least once a month, usually more than that. The cost of extra food and alcohol is just built into our grocery budget. If the party is bigger than usual, it just means that I stretch out the budget or eat from the pantry towards the end of the month. I'm also the sort of person who serves 'normal' types of food for parties. Rather than spending big bucks on gourmet food, we invite everyone we know and serve the same kinds of food that we would normally eat. For lunchtime get togethers I often make brunch foods. For dinner, I make things like tacos, BBQ sandwiches, or we grill out. I make simple side dishes as well. This past weekend we had a party and I set out BBQ sandwiches, a greek salad, a veggie tray, a bowl of cherries, and chips & dip. The food was simple, but yummy.
We also entertain about once-twice a month. Its factored into our budget. I am also a cook who just cooks what we normally cook and make enough for how many we are having for dinner.
I usually host a larger party about once a year and we serve frugal things that deliver a larger outcome. We are not into ordering in, but making and cooking our own food. So if we have our immediate family (35 plus) we cook a few beef roasts and slice for sandwiches, cook burgers and hot dogs on the grill, make chicken gumbo, lasagnas, etc. Since I am serving a full meal, we go light on the desserts and have things such as choco chip cookies etc. Usually when we are going to a party, I bring either a quick to make appetizer or bring my home made choco chip cookies that everyone loves. Actaully I have gone into homes and the first thing out of someone's mouth is- did you bring the cookies? And this can be one of the younger ones or older ones.
We also watch sales before the party. Such as, when soda and paper plates go on sale or certain items I need for the party. Or we might buy the beer, wine or liquir we will need one pay period and something else the next one.