1. Scrap Metal. You need this! Obviously, it's hard to add metal to a car that it has rusted away on without new metal!
Cost: $0
2. Welder. Hard to weld in new metal without one. I got a nice little Lincoln Electric MIG from Home Depot. It's actually a pretty good one.
Cost: $450
NOTE: You could get a TIG if you'd like, but for long welds, that can get tedious. A stick welder might work too, but sheet metal is so thin, you'd have to be really good. So go with MIG.
Wanna learn how to weld? Not to hard, just click on my little tutorial thing, that is, once I make one...
3. Time. This isn't a quick fix type of thing. It's not bondo or fiberglass, and it's a proportional relationship between time taken and how long it lasts.
Cost: I dunno, nothing? A lot?
With that said, click onto one of the links (I guess there's only one right now, but you know...)