Before you considering repairing cosmetic rust, you should have the underside of the car inspected to be sure there is no rust that would adversly affect other areas like floorboards.
As far as repairing rust, it's not worth it unless you intend to move out of the rust-belt.
I worked with a guy in Long Beach, CA who repaired rust properly, which entails cutting out the rusted area and welding in an entirely new piece of sheet metal. He only repaired high end cars like '55 T Birds and Porsche Speedsters. If you asked him to repair the rust on your '93 Dodge, he would have counseled you against the idea.
Because the new sheet metal was welded in, it's not protected from rusting out again because it cannot be rust-proofed on the inner side. It rusts out even faster than the original metal did the first time.
The cheap way to repair rust is to bash in the rusted edge, mig in some metal in the general shape and bondo it over. This kind of repair leaves you with a real mess when the rusted metal that wasn't removed continues to rust out.
Generally speaking, people who have lived their whole life in the rust belt have their new cars undercoated, drive their older car in the winter and spring, and get more years out of the new car by only driving it in summer & fall and garaging it in inclement weather.
If you really want to go bargain basement, you can get some stainless steel tape at Pep Boys and apply it to the rust on your doors. But it won't look great.
