As others have noted, it's very much dependent on what's wrong, and the overal value of the camera.
Really though, I think the root of thse factors lies simply in the availability of parts. I've fixed a number of higher-end PTZs in the field, made possible only because the parts were readily obtainable. One of the Pelco Spectra lines (III?) was notorious for frying their power boards... however, it's a module that could be easily obtained and swapped in by even a semi-qualified service technician, in the field, without even having to remove the camera housing from its location.
More and more cameras, though, are pretty much homogenous units, and even IF you can determine and remove the dead component, good luck actually getting a replacement part, unless you have other dead cameras of the same model for "transplant donors".
And too, sometimes when one part fails, others aren't far behind, and repairing a camera will bring it back only temporarily - it becomes cheaper in the long run to simply replace it, than to keep coming back to fix it.