$20 - 2 bales of hay
$5 - Tableware
$5 - bag of apples
$2 - outfit at a Thrift store
$10 - candy
$5 - craft supplies
$3 - Misc. stuff
The hay is used for two activities, but if your budget is tight you can reuse the hay from the first activity. The $20 estimate is based on $10 a bale, which is the most expensive I've found hay.
Depending on the size of your party, you have two options. For a small party go to the dollar store and pick up a tablecloth (paper), plates, napkins, and cups. You can usually find them in a Halloween theme. For a larger party you will need the same supplies, but go with the plain black and orange tableware because you get more for your money.
Every year I change the craft, but I try and stick with something cheap. The craft will probably go in the trash eventually, so no need spending $5 per kid on some knick-knack. There are some exceptions to this, but as a general rule don't go overboard on the craft. Oriental Trading has a lot of great craft kits. You can purchase one or just use the site for ideas.
There isn't really anything I buy extra, but I find it is good to have some extra money in the budget to cover unexpected details. Perhaps you could use it for a bag of chips and kool-aid. Typically I have my parties at 2pm so I don't have to worry about feeding people. For $3 you could buy a box mix of cake and icing, make cupcakes, and still have some change left over for kool-aid.
My formula for good parties is to have: 2 games/activities, 1 craft, and a means of getting mass amounts of candy/toys/misc. at the end.
After all the guests arrive I start with the classic game of bobbing for apples. The best way to play is to have a plastic tote that is long, and only fill half way up with water. Also make sure you have towels lining the floor and a wipe off towel available for the kids.
Next we go outside and I show the kids the bale of hay and a pile of clothes. It is their job to build a scare crow. Usually I have a chair outside for the scarecrow to sit on. For the head I use our jack-o-lantern, but if you don't have you can get an orange plastic trick-or-treat pumpkin for less than a dollar. It is nice to have the kids build a scarecrow for my front door, makes a great decoration for trick-or-treaters.
When they are done I have them come inside for the craft. As I said earlier, I vary my crafts, so this gives you some freedom.
While the kids are working on the craft, I have another adult (usually my husband) go outside and scatter half of the second hay bale in a big pile. Then they toss candy all over, sometimes I add some goodie bag toys I've collected over the year and sometimes pennies too. Then the other half of the hay is scattered on top. I give the kids grocery bags or brown lunch sacks to turn them lose to find stuff.
If time still remains we play a game to run the kids energy out. The original game is called Torpedo (in case anyone has heard of it), but we change it to suit our needs for every party. For this purpose we will call the game Pumpkin. You tell the kids the left side of the yard is the trick and the right side is the treat. You will shout out directions (either trick or treat) but when you yell Pumpkin everyone has to touch the floor. The last person to touch the ground is out. The last person standing gets to be the caller.