It sounds like you want something that will supply 5V to a USB port. Even f there are technological differences, I would guess that the biggest difference in cost (kw-h/$) would depend on whether you but at retail or with a discount, and not on the brand or packaging cosmetics. My only concern would be that ones with a high energy density (kw-h/kg) might spontaneously catch fire, and I do not know of any reviews that address this issue (I think these would all be Li-iion, and you are not allowed put these in your checked baggage). I would get one in the $10-$20 range and test it out at home, and maybe buy another one if it does not have enough capacity (do you know the discharge rate for the tablet?).