This instructable will show you how to take a $12 Prince of Persia plastic prop (say that three times fast! Ha!) sword and turn it into something you might actually want to accessorize your pirate costume with.
I had most of this stuff in my crafty closet. You may have to make some purchases. This whole project cost me about $20, but if you have to buy everything, it may look more like a $50-$75 sword/dagger/cutlass, in which case, you might do better to buy a replica anyhow.
The inspiration for this came from TribalDancer and her turorial on modding a pirate gun. Check out her 'ible here: http://www.instructables.com/id/Steampunk-Pirate-Gun-Mod/ . If you want to know what kind of awesome costume either of these accessories could adorn, you can check out a story about my pirate costume on my website, www.ericayoung.com. I'm pretty new around here, but I just finished my first Instructable here so check it out, too: A steampunk necktie corset dress for all (okay, maybe several) occasions!