Gday Mate.
If you can find a cheap door, depending on the amount of rust in yours, buy it. Replacing panels is a lot easier than fixing them
As to the question regarding cost of welding in new pieces, the cost can be as long as a piece of string. All depends on location, size, how much time is necessary to manifacture the replacement panel (bending it etc).
For example, i had a lot of rust behind my windscreen, and it was pretty straight forward to fix, just cut out the rust, make up a new panel and weld it in. However i then started fixing rust around my rear lights, and it took me significantly longer to bend the panel up to get the right shape.
If you intend on doing all of the rust repairs yourself, you will need to invest in some tools. Stuff like a angle grinder, protection gear, files, sanders (electric and block), a decent welder (mig is the best for body repairs, but also the most expensive) and a whole heap of other incidentals. If you get the panels welded in at a shop, they will of course charge you a hefty fee for labour.
Post up some pictures to give us all a idea in regards to the severity of the rust, and perhaps we can give you a better idea of prices and replacement advice.
Cheers, hope that helps,
Justin