Stephen.
Whether the prices you mentioned are reasonable or not is irrelevant. The fact is those are the going rates, so if you can not, or will not, pay them the only option I see for you is to learn to fix the clutch yourself.
Naturally that will involve purchasing some expensive tools and equipment, then attending some sort of training course so you know what and how the tasks are performed, and finally you will need premises on which to do the work. Does that put the garage costs into perspective?