I wanted to get a spare; checking online told me that the Apple Store wanted US$29 for the little plug, which just seemed like too much for too little, so I went shopping.
On eBay I found a bunch of auctions based in Hong Kong selling what looked like the exact same thing for anywhere from under around $2-4, and they often included the USB cable to boot.
I ordered four from a vendor and after a few weeks received them. They looked perfect, right down to the little green dot -- but none of them worked. They might start charging, but after a few minutes the iPhone would beep a few times and I would be informed that "this accessory is not compatible." And so it went for all four pieces. After sending some pictures back and forth and reading a few emails containing a combination of surprise and amazement from the vendor, my money was refunded.
Undaunted, or maybe just dumb, I went back to eBay and found another vendor that claimed to be selling, "100% Brand New Original OEM Charger and Original OEM USB Cable for iPhone/iPod" for around US$4. I just checked eBay and found this same vendor raising the price to US$10 in one auction and lowering it to $US.01 in another.