Thanks to the XDA developers forum and HTC Source Codes, Now it's possible to convert your regular "Sense 6" HTC One M8 to a Google Play Edition M8 with Pure Android UI without paying that extra $100. The Full Guide is on the Source link below, proceed at your own risk.

A few things you should know before you proceed. First, you will need to erase the entire internal storage (NAND). To do so, you'll have to bypass HTC's security with a special tool. Then you must unlock the bootloader in order to flash a custom ROM. There is a step-by-step guide for the entire process, but it is not easy, and there is a chance you brick your HTC One M8.

Source (XDA)