The average cost of a Darth Vader costume is $3000-$5000. Crazy, isn't it?
To further add to the cost, today I picked up Vader's cape and tunic from the dry cleaners. The total cost of cleaning the two items came to $20.50. Ouch.
Being a plain old stormtrooper is much cheaper. The costume is simpler (although still not inexpensive), and everything that isn't plastic can be tossed into the washing machine. No dry cleaning necessary. I think I much prefer that!