How much power does a GPU actually use when only doing PCIe I/O, or only compute?
In the lack of manufacturer disclosure (most consumers of pcie modular cards don't care about power usage, though embedded pcie devices might in order to beat maximum power draw numbers in the nano board  or thin client market), emperical testing is required.

This can be done by identifying the power rails (PCIe pin out), and interfacing an ammeter inbetween them. For most purposes, pcie has multiple 12V and GND pins, so you might need multiple ammeters. Cheaper ammeters will average the current use out, good enough for most purposes. (Ex, showing 200mA at 12V, over 10 minutes with a steady load of 10% would mean 0.6 W average).  A scope or logging output would allow graphing for more detail.

You could hack a motherboard up to access the traces, but a spare extension card (riser or dual adapter) or cable would be useful if you didn't want to take a dremel and soldering iron to a full board.