what about using a RC battery like a Turnigy? They are much cheaper, you can easily make up some jumper cables to jump the bike or your buddies bike.
Something like this? I have no idea if it would work but these RC batteries put out an astonishing amount of power. I don't think the amp hour really matters since the starting cycle is so short.
http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html