The Star Chief/Executive was the low-priced trim level for the full-size Pontiac line. To elaborate, the mid-priced, full-size Pontiac was the Catalina, the high-priced model was the Bonneville and the top-of-the-line, premium model the Grand Prix (the Grand Prix became a mid-size model in 1969). I once knew a person who owned a 1965 Star Chief 4-door sedan back in the early 1980's. I remember seeing Pontiac Executive's (without the Star Chief designation) in the late 1960's and early 1970's.
I am sure it is a very nice car in its own right, but from a collector's point of view, it may not be as desirable as a fully loaded Grand Prix hardtop, Bonneville convertible, or Catalina 2+2. Now there are people out there who have an attraction to the low-priced models. You just have to work hard to find them.
For a good "ballpark" estimate of value, get your hands on the latest "Old Cars Price Guide".
Good luck with your Pontiac.