I dont believe cheap is the way to go with electronics. I bought a German device called an E-werk. The cheapest I found them was in Germany via the websites of Starbike.com or Bike24.com.
Peter White Cycles has plenty to say at http://peterwhitecycles.com/ewerk.asp
I use the optional cache battery and have had no issue with using it. It keeps my gps and my phone charged and my dynamo hub lights up my world at night courtesy of my Edelux headlight. I only charge one thing at a time due to me being too lazy to work out how to adjust to a bigger current which the E-werk can do but I cant be bothered with more reading of manuals. There is nothing worse on a tour than sitting in a caravan park laundry or bathroom for hours on end trying to charge your phone or gps etc and having to sit there unable to leave least someone pinches your gear. Best investment I've made yet I reckon.