This may help. It's not exactly what you wanted to hear but it's probably better than what the camera shop has advised.
How to remove a stuck Canon 50mm 1.8 lens from your camera
Note that this may not be what has happened in your case, but it sounds very similar.
He says -
- What happens is that there is a snap on plastic dust cover on the mount end of the lens. The dust cover is basically a plastic ring with four fingers that snap into the back of the lens housing. The cover encompasses the inner lens and also surrounds the electrical connections on three sides. Hopefully you are starting to get a picture of the part I am talking about. What happens is that one or more of the fingers that snaps into the back of the lens breaks allowing the dust cover to fall slightly back towards the camera body. When you go to rotate the lens to remove it the lens only rotates maybe one quarter of a turn and will not rotate any more. The plastic dust cover has snagged on the cameras electrical connection.
His fix is potentially destructive of the lens and not of the camera if care is taken. There is a possibility of saving the lens - and more so if you understand properly in advance what you are setting out to do, what is involved, why it works and what damage is likely. So read the (short) article carefully first. If you are not confident with your mechanical competence you could consider showing this 'fix' to a person who is used to fine mechanical work and ask them to try. I'd expect that a competent "watchmaker" would be likely to do this successfully.
It is also possible that, if this is what is wrong in your case, you may be able to plan an "attack" from the outside with pieces of material that allow you to attack the jamming points - perhaps mylar sheet or steel or brass shim material that you can cut "fingers from to slide into the lens-body interface. A mechanical "feeler gauge" set may provide suitable material. If trying this method, take the camera battery out first. Bearing in mind, of course, that damage to the lens is potentially acceptable but that damage to the camera is to be avoided.
If I was attempting this I'd read as many of the descriptions of the frontal attack method that I could find and look at the photos (see 'useful page' below) and THEN see if a feeler gauge attack may be able to be planned. That's just me, and you may be better advised to not even try this. If it does work it probably has more chance of actually recovering the lens than the other approaches. But, who knows what you might do to the camera in the process?
As a warning about brute force, he also notes:
- Some people said to just twist hard and the lens should break and come off. Others said that they twisted hard and the lens came off but it wiped out the camera side of the mount, meaning the electrical connection pins. The local camera repair shop had no idea what I was talking about so I decided to give it a go on my own.
The link from Inkista contains a link to this useful page of people's accounts of experiences with this lens. There are similar suggestions to those from the site above. All seem to suggest that you need to 'go in' from the lens front and I did not note any suggesting my 'feeler gauge' approach.
How NOT to do it