Guess you've never heard of Alternating Current (AC) TIG welding? It handles the aluminum oxide problem rather nicely, in my experience. The real problems with aluminum are: 1) it conducts heat very well; you need a lot more power to weld aluminum than to weld steel and you have to move quickly when you do it, and 2) there's a very, very fine line between having the metal hot enough to weld and so hot that you burn through it. This is especially true with thin aluminum; one second it looks fine, the next you've burned through it and you're looking at nickel-sized hole in your tubing.

To the OP: I would expect that having an aluminum bike repaired would be expensive. Not many people know how to weld aluminum well. Of those people who do, there's a smaller number who have the skill to weld thin aluminum. Those people are usually in demand and their services don't come cheap. In addition, as ElJamoquio points out, you may need to have the frame heat-treated after welding. Finally, alignment is critical for a bike frame. A general-purpose welder may not have the jigging necessary to properly align the frame while the repair is being completed.