Star Trek manuscript/screenplay

[Dennis]

I just found out that my "almost" son-in-law has an original manuscript or screenplay for Star Trek, signed by his uncle Gene Rodenberry, apparently it was a gift given to him before Star Trek became so big. Unfortunately the boy never cared for his uncle Gene or aunt Majel so he may just give this away to the first person who asks for it.
My question is just how valuable is some thing like this, anybody have an idea?