welding aluminum is difficult, because the aluminum oxide (surface layer) melts at 3500*F while the actual aluminum is around 1200*F, so the oxide must be removed (it doesn't stay gone very long) and welding it can be a pain the but. if you can find a local shop that would do it, it shouldn't cost much.
if the break is at a joint, it can probably be fixed, but if a tube is cracked, i wouldn't ride it.

pcad tossed a seven (right?) frame out in the trash because it cracked and its too hard to weld ti.

edit: cost: if you get a bike shop to repair it, it'll be loads, but if you go to a small fabricator or auto shop they can do it in a couple minutes.