A good likable Cadillac in the range you're looking for is probably going to be more than the bluebook by anywhere from $500-1500, as there is a growing demand for these cars. That's why we get so upset when they get wrecked and insurance companies want to write you off. You simply cannot get a great shape older Cadillac for the price the book says. Unless you get a little old lady who is more likely to budge (as they see the car as transportation rather than a collector item).
A decent '94-96 "whale body" RWD Cadillac is gonna be anywhere from $3K-6K, depending on miles, etc. Low mileage ones or mint ones may be slightly higher.
A decent '90-'92 is anywhere from $2K-$5K if you get a 5.0L engine, $2.5K-8K if you get the 5.7L. Clean and low mileage ones are known to go over $10K, though.
A decent '86-'89 is roughly the same range as the cars above. Slightly less, though, because the 5.0L engine was the only engine, carbureted, and the front end is less desirable to some.
An ok '82-'84 is gonna be hard to find since the 4.1L engine was a piece of junk (according to others). Even a nice one is anywhere from $500-$2000. Only do these cars exceed that range when they're heavily optioned, or they're so mint, they're a collector item.
An '80-'81 is gonna be the most variable in ranges, because the big block 6.0L engine is highly desirable, but being older these cars have more miles and problems, and cosmetic issues are more prevalent. One that is in decent shape will run you anywhere from $1500-3K. If in mint shape these cars will likely exceed that.
That's just a rough run down of what I've noticed. There will always be exceptions. If there's another engine like a V6 or a diesel that will affect the price. Also clean cars tend to bring in a pretty penny, unexpectedly. Also if the car is a coupe, add about $1K-2K to the price, as coupes are generally more desirable.
Now these are RWD cars that I have mentioned. After the 1985, the Deville became FWD, but I would only look into ones from '91 and after, since the engines before then weren't so great. I know the 4.1L was a dog, I'm not sure about the little 4.5L, but I know the 4.9L that was in cars from '91 on is known to be a great car up to 200,000 miles with regular maintenance.
Good luck on your hunt!
Greg