Sure if you can get a pole at that price including shipping, it's a bargain.www.music123.com
sells 8' flag poles at that price, with free shipping and no minimum order. It is
But I'm also not in total agreement with all the pvc hate myself. It's a great blue sword core for new fighters who are still learning. I'm not sure if it's worth it for red weapons, though, when alternatives only cost a few dollars more.
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Back to the original question, though - you can try putting things inside the pvc to reduce the flex, but this often won't work very well. If you add wood inside the pvc, it might reduce the flex at first, but the wood will soon start to crack and - once it breaks - the sword will flex even more than it did before because of the added weight inside it. The only thing I can think of that would reduce the flex successfully would be fiberglass; but if you have fiberglass available, you should use that instead of pvc, not in addition to it.
The best practice is to use pvc thick enough not to flex too much from the start (rather than trying to make a floppy core stiffer), or to invest in a fiberglass core that doesn't flex to much.