If you want to make a mermaid's tail on an incredibly tight budget, and make it look the best you possibly can, first get some painter's tarp. Cut the tarp to wrap around the actor's lower half and legs. Taper the cut of course, and cut the top portion to fall below the actor's belly button.
Then I would suggest Ace's idea of liquid latex. If you use it sparingly, a bottle ( approx. $ 14.oo ) goes a long way.
Get a can, or cans, of Pringles potato chips. Take the lid and use it as your scale template. Pour a thin layer inside of the lid and let it dry. Once it drys, powder it wit some baby powder, remove it, and powder the other side (this will ensure that it won't stick to anything on it's own). Then cut the latex in half, and you will now have two scales to work with. Get some lackeys to help you and you will have enough scales to cover the tarp in no time !
Airbrushing will look better in the end, but for a budget, use spray paint. They have all sorts of colors. Even metallics. They're cheaper than airbrushing if you don't have your own gun and compressor + paints.
For the tail, cut the desired pattern out of the remainder of your plastic tarp. Cut out two identical patterns. Cover the two tail patterns in a thin layer of the liquid latex. While it is wet, you can play with striations, or designs, etc. If you want some areas thicker than others, keep applying fresh layers of latex to the dry ones. You can make incredible tail striations with a steady hand.
Once you have both halves of the tail complete, glue or latex them to the scale covered tarp. Once the two sides are fused together, you can stuff the tail with tissue paper however thick you want the tail to be.
This may seem like a bothersome process, but once you get going on it, it really takes no time at all. And you will save big $$$$$ in the process.
When you are ready to fit the actor in the tail, you can latex the tail shaft directly to the actors skin using spirit gum adhesive and a stiple technique. The wet latex will pick up on the color of the scales, and as it dries, it will automatically blend the actor's skin tone into the color of the tail. Cool huh ?
Well, I hope my idea helps. Good luck with the film. An if your ever in California and need someone to read a script for you. Don't hesitate to to call ol' Cut, aye ?
