Measuring current draw from a USB port on a PC can be a little problmatic from a practical side. That's why I jumped on the below cute little item a few months ago when I came across it. Seems to work ok but not the most rugged thing I've come across. Alternates display from volts to milliamps and I haven't verified it's accuracy yet, but at the price I couldn't resist.
http://www.ebay.com/itm/390646012397?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT