I got a very small, 6 inch USB extension cable from my favorite cable store. Then, I chopped it up to get access to the power supply wire and clamp my cheapo ammeter on to it. I plugged a card reader into my PC with this contraption. Note that I'm measuring the USB power supply at ~5v, not the 3.3v that the card actually gets and that these are very unscientific measurements made with Radio Shack equipment.
My card reader alone: ~31 mA (unchanged if a normal SD card is simply plugged in)
After plugging Eye-Fi card in (no pending uploads): 56 mA (25 mA for the card)
All the numbers that I'll state from here on include only the amount that the Eye-Fi card is using after subtracting the amount for the reader.
Then, I took a picture to make the start start an upload and turn on its radio. The ammeter was bouncing all over the place, but it peaked at over 170 mA and I'd guess that the card averaged about 120 mA during the upload. The strange part is that, after the upload, the power usage only goes back to 40 mA. It never (at least in a couple of minutes) gets back to the 25 mA where it started. I wonder if the radio never gets completely turned off. Hmm.
My Nikon D50 has a 7.4v 1500 mAh battery. Let's be stupid and pessimistic and assume that the card is running at its very peak power of ~170 mA and round that 5v USB voltage up to 7.4v. That's still almost 9 hours of theoretical power. I can't possibly be seeing that effect on my battery life, right?
Is my methodology screwy? Any electrical engineers out there that want to tell me how much of an idiot I am? Email me.