I would be careful with valuations. Some dealers would tell you it is worthless, buy it from you for very little and then try and sell it for hundreds.There is a score signed by Sorabji on e bay at the moment which is for sale @ $2500.
I'm assuming it's an autograph, not the manuscript.
What if you donate it to a university/conservatory or some place that will take care of it and let everyone still have access to it?
Is there? What's the link?
https://www.abebooks.co.uk/Opus-Clavicembalisticum-Sorabji-Kaikhosru-Curwen-Sons/11807060728/bd It seems to have vanished from e bay, but I think this is the same item listed on Abe Books.