I'll show you how to make batman vambraces on a VERY low budget! And it works fine! Looks good! Worked for me at this year's Halloween party! :D cheers! All you need is construction paper and some tools!

Step 1: The Materials and Tools.

You'll be needing
1) Black Construction Paper
2) Craft glue
3) Staples
4) scissors
5) Tape (optional..but keep it, just in case you need to repair any torn bits)

Step 2: Making the Basic Shape

Take a piece of black construction paper, as long as half of your forearm and as wide as about half of your wrist (or maybe a bit more, depending on how you would want it to look). Now fold the paper in half, and cut out a basic shape, tapering towards the upper part and broader towards the lower part. It'll look like this.

Step 3: Making the Fins.

Now take a fairly large piece of construction paper and fold it to make a square. Cut out a fin shape keeping some extra bit of paper at the bottom (you're gonna need it to stick the fins). You'll be having two identical fins. Now cut 'em out and separate them! It'll look like this.

Step 4: Sticking the Fins

Fold the bottom of the fins, where you had left the extra paper, to form a rectangular base. Now glue it along the middle of your Gauntlet!

Step 5: Making the Armband

Now you have to wear it. How are you gonna keep it in place? You'll be needing armbands! To make an armband, cut out a strip of construction paper, a rectangular one. Now Staple one side of it to one side of your gauntlet! Then cut out a slit on the other side of the gauntlet so that you can slip the armband in! And voila! There you go! :D

Step 6: Wearing Them!

When you wear them, just slip in the armband through the slit and adjust the tightness! And there you have! Doesn't even fall off! :D
I made a pair for my batman costume! :)