Here are three sources that might help:
When finding the value of older vehicles, beauty is in the eye of the beholder or key holder in this case. Most price guides for late-model vehicles is based on "dealer only" auction prices or retail prices paid by consumers (DMV title information).
Since very few 1959 Cadillac cars are bought and sold in a given year the data from these guides can be misleading. If the car is rare, it might be in your best interest to have a skilled appraiser look at the vehicle to determine the value. In my opinion, a classic vehicle is worth what the individual buyer is willing to pay.
Editor, Brake & Front End magazine