Is there any way to evaluate what I could expect to get for a certain make, model and year of a car that’s good only for parts? My mom’s 1994 Cadillac De Ville just breathed its last, and a salvage guy is offering $100.00 for it.
Are you looking to sell the car as one piece or part it out and sell the parts yourself? If you want someone to take it off your hands as a whole car, $100 for a dead Deville is perfectly fair. You could get many times that if you sold the parts separately, but you would have to dismantle the car, take pictures, clean parts, store them, and deal with selling them all. It all depends on how much work you want to do. 
ETA: It will also depend on what killed the car. If it all blowed up, $100 is fair. If it died because a transmission died or something, you might get more from it. You would still have to advertise, store, etc., though. Most people find it easier just to dump it to a salvager.
I’ll second what Necros said. A friend of mine and I are parting out a couple of cars on eBay and it fascinates me to see parts go for a few bucks and others causing bidding wars. (My friend finds tearing the cars apart very therapeutic, and he’s got a place to work on them.)
Having said that, if you have any inclination to go that route, spend a little time searching the completed listings on eBay for parts from a '94 De Ville. You may find that the prices parts are selling for aren’t that great, or that there are so many resellers of new parts that used parts have no market.
I got a couple hundred dollars in the mid-'90s for a Subaru that had been crushed by 3 oak trees. I had just replaced all CV joints and tie rod ends, and put that in the ad. The engine part of the car was not damaged, but it was very clear that the car would never be drivable again. Take that data point for what it’s worth.