Hmmm. I could have sworn that the last time I looked this one up there was a hyphen in the adjective form. Nope.
Webster's New World, Merriam-Webster online, Dictionary.com, and American Heritage all list the adjective as one word, no hyphen. It means, primarily:
Supplied, installed, or purchased in a condition ready for immediate use,
occupation, or operation: a turnkey computer system; a turnkey housing project.
I don't like it as one word. It's too close to "ternkey" -- a new Thanksgiving entree I just made up that could be a surprisingly affordable alternative for anyone who lives near a beach.
(Yes, I'm kidding. Please don't call PETA on me.)