redo's would likely cost more because you have to remove all of the filling, and then basically perform the root canal from scratch.
Although likely not the case, short fills can be characteristic of the use of a material commonly known as "sargenti paste". They also have high failure rates. Sargenti Paste contains formaldehyde as its active ingredient and lead and mercury. It can cause all kinds of problems, immediate or delayed. Go to an endodontist (not general dentist) and have the original root canal evaluated. If it is Sargenti Paste and were my tooth, I would have the tooth extracted.
Actually, usually a root canal retreatment is more expensive than the initial root canal I think.