Although this has been authenticated by two separate authenticators I'm not totally convinced. The inscription looks more convincing than the signature (both the "Marilyn" and "Monroe" look a bit atypical to me) so maybe I'm being over-cautious. If it is right it looks like it should date from later than 1952. Again, maybe I'm mistaken here.
In any event $6,500 seems a pretty full price for what is essentially a signed-on-paper autograph. I think you'd be able to pick up such an autograph for a couple of thousand less at somewhere like Heritage or RR Auction.