Cheap, relatively easy to make if you have a hot glue gun, and it can be made in very little time! (Two or three days for me, one day if you having nothing else to do)
Step 1: Get Your Materials
This is a flexible costume, so it's fine to change some of the materials for a different look.
You will need:
-A sturdy full face mask (can be found at hobby-lobby for $4. I messed around with mine, cutting extra nose and mouth ventilation)
-White polymer clay (I used sculpeyIII, which came in a $9 block and bakes in the oven.)
-Acrylic paint (a couple dollars)
+Black +Skin color, or red, blue, brown, and yellow to make your skin color +White for touchups on the teeth
-A hoodie (Optional--a wig can also cover the elastic bands, or a beanie hat, or whatever you want! I already had one, so I didn't figure it in the cost)
-Elastic bands or thin headbands ($2-3, not pictured in this step.)
-Black burlap or another black mesh to cover your eyes. (Make sure you can see through it!! However, a very creepy effect can be achieved by letting your eyes be seen from inside the mouth as well. Your call.)
-Hot glue/hot glue gun
-1" thick Upholstery foam (Optional, to make the mask compatible with glasses)
Step 2: Decide on Teeth and Placement
I decided on big, pointy, terrifying teeth, but you might decide a big set of human teeth or wolf teeth would be creepier. Maybe fish teeth?
Whatever you decide, put on the mask and figure out where the "lips" should be, then draw the placement of the teeth bases on your mask.
Step 3: Make the Teeth
After conditioning your clay (kneading it till it's soft), roll two same-sized lumps of clay into spheres. This will make sure the teeth are similar is size. start at the center of the top row of teeth and work outwards making teeth, two at a time.
Roll each ball into a long pointy cone and then bend it into a nice curve. Smush the fat end onto your mask to make a flat base to hot glue on later. The clay is fairly light, so you don't need a massive base.
Once all the teeth are done, carefully pull them off the mask and set them on their sides on a pan, in the correct order. Bake them according to the clay package instructions, but I would recommend baking for at least 30 minutes.
After the teeth have cooled off, sharpie the location on the tooth on the base. I used T and B to denote top and bottom, and then a number for the position left-to-right. Seriously, do this. They will all look the same if you jumble them and they all have a slightly different base.
IMPORTANT NOTE: If you're making this for a child, don't make the teeth sharp!!! They seem soft when you make them but they do harden into something kind of dangerous after you cook them.
Step 4: Optional Glasses-compatibility Step
Cut your foam into pieces similar to in the pictures and glue them inside the forehead and cheekbone areas, just test them before you hot glue to make sure you're not pinching your nose with foam or hitting your glasses.
I had to glue an additional half-thickness of foam on top of these because my glasses are extra wide and huge, and thus were jamming into my face with the mask on.
Step 5: Inside the Mouth
First paint the area inside the mouth black--this keeps it from showing through the mesh or burlap.
Next, cut a rough shape of the black area from your fabric and hot glue it over the eyes, nose, and mouth. The teeth will cover up your rough edge, but don't actually get the fabric where the bases of the teeth will be glued. They might not sit correctly with thick fabric under them, or have a good grip on the mask.
Step 6: Attach Teeth
I forgot to get a picture of this step, so these photos are actually the unbaked teeth on the mask, but you get the idea.
Hot glue the teeth where they should go, again working from the center to the edges. Once the teeth are all on, start carefully building up hot glue around the edges to form "lips", or make it look like the teeth are really coming up out of the skin. I used a popsicle stick to pull the warm hot glue away from the edge to make a rough blend into the rest of the mask. You'll see it in the next step.
You could make gums AND lips this way if you wanted, but you might start melting your mask with all the heat at once. Better to use clay or something for that if you wanted to.
Step 7: Paint!
Here is where you really want to get as close as possible to the skin tone of your neck. If you're mixing the color, and have light skin, I would start with a yellow-y color and work in red, with a little blue and brown. If you have dark skin, it may be easier to find your color already mixed or just add blue, red, and yellow to a lighter brown to adjust it down.
I suppose you could use your own makeup if you have it, but I'm not sure how well it would stick and it might be messy.
Touch up any mistakes on the teeth with white, or just make it a gore fest and splatter them and the gums in dark red so no one will notice. :)
Step 8: Elastic
That little elastic strap that comes with the mask probably didn't last through you trying it on. I bought skin-colored headbands at walmart and hotglued/tied them into the thing in the pictures. Remember that whatever elastic you use will stretch out a little, so make it slightly tighter than you think you'll need. Use plenty of hotglue to hold them on inside, but be careful not to melt your mask.
Wherever you make a knot, hot glue it to make sure it stays.
Step 9: Terrify Everyone.
You're done! Now pop on a hoodie or beanie and head to that Halloween party.
I added two black plastic sickles from walmart for funsies, but a "Free Hugs" sign will work just fine, especially in a no-weapons-allowed place like school or work.
Also, don't be dumb. The teeth are sharp, and can hurt people.