The range on an ES probably goes from < $500 to about $6500. Possibly more for a truly special car. Of that value my rough guess would be:
60% - body condition - rust and crash damage/repairs or lack thereof
20% - interior condition - dash, leather on seats, rubber seals
20% - mechanical condition - the easiest for most of us to fix.

You really have to see the car in person to get a feel for how nice it is. While they are rare - they aren't so rare that a dog is still worth a lot - they are no Ferrari. And the ES wagons aren't worth quite as much as the coupes - despite the greater practicality. In the end the price is what the market will bear.

Check on eBay - there are usualy a fair number of 1800's of various conditions and years going under the hammer there - look at the pic's and prices to get an idea of how much others are willing to pay for one currently.