It can be done pretty cheaply: http://www.scubaboard.com/forums/do-it-yourself-diy/354001-tank-equalizer-2-50-a.html?highlight=whip
But not very quickly if you are talking about moving larger volumes of gas. Best used to put a puff in an emply tank or top off a pony.
It works more quickly than I would have expected, but the price has sky-rocketed to almost three dollars.