Is there a site where I can find out what some movie scripts sold for?
Some sites require registration, and others are pay sites. (Such as hollywoodreporter.com, one of the most trustworthy in my humble opinion.)
If you’re looking to find out how much a specific script sold for, try Googling the writer’s name or the project’s title, or search inside Variety.com. But be aware that the sales price is often not reported, or when it is, is often wildly inflated.