ok, use a 3G modem with its own battery pack. or, why not have a bluetooth 3G modem in the same form as a bluetooth GPS. besides, 3g modems are getting more efficient, so lets set the limit to say 150mA?
it doesn't have to be PCIe-x1, it just needs to be fast enough to allow you to add more than just a mass storage device or a serial adaptor sort of thing (gprs/3g modem), e.g. video adaptor wotsit.