The taxes will be calculated on RT NR tkt, based on from which airport you depart back to States. CDG is not super $$. FRA is very high--about $130. No clue about DUS. AF zeds are low--same with AMS, but not sure what fees are for AMS. I'd poke around myIDtravel and get an idea. Train could be pricey, compared to zeds. Have fun working all the angles. Are you trying to travel over Christmas week?