Make it a potluck
Potlucks are a great idea for any party. For starters, you do not have the work and expense of creating multiple dishes yourself. You also get the advantage of trying new and different recipes. It is often easiest to make the main dish yourself, and assign dishes to different party goers (for instance, asking one family to bring a side dish, another dessert, another salad, etc.). You can also make your Halloween party snacks or desserts only.
Plan ahead and shop on sale
If you host an annual Halloween party, paper supplies and decorations can often be bought cheaply in November for the following year. If this is your first year hosting a Halloween party, start buying as soon as Halloween merchandise starts hitting the shelves so that you can take advantage of the lower prices and better selection.
Look for items that can be used more than once
If you can get two uses out of one item, you have saved money right there. Pumpkins that are painted or left natural (no carving) can be used to make baked goods after the guests have gone home. Fall decorations can work just as well in November as in October, provided that there is nothing specifically Halloween about them. Also look for Halloween decorations that can be reused from one year to the next. That way, if you do invest much money this year, you won't have to next year.
Small and intimate beats large and loud
While a large party involving the entire neighborhood may sound like fun, the amount of money that it will cost, let alone the amount of room that you will need, can make this a bit prohibitive. Consider making the party for adults only, or limit to only the families that you are closest with.
Shop at warehouse stores
Buying in bulk is often less expensive, especially when you need to buy a large amount of items. Candy, paper goods, and treats for the goodie bags can often be found for less than you would pay at the department stores.
Chances are, if you spend more money on your Halloween party than you were intending, it is only going to stress you out. So by shopping smart and planning ahead, you can have just as much fun without the stress.