Campus111 via eBay offers the HID 4300K Xenon Headlight Conversion Kit for $19.99 with free shipping. That's the lowest total price we could find for any size of this bulb in this color by $15. It's available in sizes H1, H3, H4, H7, H8/H9/H11, 9004/9007, 9005/HB3/H10, 9006, 880/881, and H13.

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Of note, it's also available in 6000K, 8000K, or 10000K for $1 more.