Hello and welcome!
You are right, in that you might have an expensive problem.
Or not.
I've not personally experienced it but there a few common causes of "being stuck in gear".
Using the principle of Occam's Razor look at the simplest solution first: gear linkage. If this is the problem is could be an easy fix.
At the other end of spectrum something has broken off inside the gearbox and is blocking movement. That would involve taking the gearbox out of the car (might mean the engine has to come out as well), opening the box, stripping and repairing. If you have the tools, workshop, and knowledge you can do this yourself, however if you are relying on a professional expect the bill to hit £1000 for labour and parts. More if a new box is needed.