By the time my sister was born, Family Affair was off the air, so I have no personal frame of reference for Mrs. Beasley.
Frankly, she seems awful Chucky-like to me, ["You may call me Mrs. Beasley, would you like to play?" vs "Hi, I'm Chucky. Wanna play?"] but suit yourself.
The eBay is the place to go for Mrs. Beasleys, of course. Original wireframe glasses and the original box seem very rare, and are the main source of collectible value. For actual playing, a talking doll in good original condition can easily go for over $200. A non-talking version runs about half that.
update via daddyzine's Garrett: apparently that Mrs. Beasley was one of eBay's killer apps. [theonion.com circa 1999]