Catnap 

There’s two kinds of repair - to the components or to the frame. In either case, if you’re doing the work, labor is free. If it’s done at a bike shop, consult them on the labor cost.

Gauging the repair cost for components is easier - you do research to determine the cost to replace the broken components, either with the same parts or new parts.

Frame repair is more costly and harder to do, as it takes more specialized knowledge and tools. Generally, unless the frame is of very high quality or especially valuable, major frame repair is not worth the effort. Dents are the easiest to fix, while structural damage is generally the hardest. Damage that compromises structural integrity, like crash damage, cracked joints, rust holes or bent tubes, will necessitate major repairs costing in the hundreds to thousands of dollars. It’s essentially like building a new frame, but with all the challenges of dealing with an old frame.

Generally, bikes that have suffered crash damage to the point that it deforms the fork or the frame (or both) are not worth repairing and not safe to ride as-is.

An “in-between” subject are issues like a stuck bottom bracket, stuck stem or seatpost, or ruined threads in the steerer tube or bottom bracket shell. There’s zillions of tutorials on how to remove stuck components - either you do it yourself and put a price on your own time, or you pay a bike store. Ruined threads might be repairable by a store, but in some cases fall into the heading of frame repair.

there’s many twists and turns beyond this basic approach, which other folks exploring those in this thread. But this was my attempt to answer your question as literally as possible.